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| <div class="section"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="intrusive.auto_unlink_hooks"></a><a class="link" href="auto_unlink_hooks.html" title="Auto-unlink hooks">Auto-unlink hooks</a> |
| </h2></div></div></div> |
| <div class="toc"><dl> |
| <dt><span class="section"><a href="auto_unlink_hooks.html#intrusive.auto_unlink_hooks.auto_unlink_hooks_what">What's |
| an auto-unlink hook?</a></span></dt> |
| <dt><span class="section"><a href="auto_unlink_hooks.html#intrusive.auto_unlink_hooks.auto_unlink_hooks_example">Auto-unlink |
| hook example</a></span></dt> |
| <dt><span class="section"><a href="auto_unlink_hooks.html#intrusive.auto_unlink_hooks.auto_unlink_and_constant_time">Auto-unlink |
| hooks and containers with constant-time <code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code></a></span></dt> |
| </dl></div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="intrusive.auto_unlink_hooks.auto_unlink_hooks_what"></a><a class="link" href="auto_unlink_hooks.html#intrusive.auto_unlink_hooks.auto_unlink_hooks_what" title="What's an auto-unlink hook?">What's |
| an auto-unlink hook?</a> |
| </h3></div></div></div> |
| <p> |
| <span class="bold"><strong>Boost.Intrusive</strong></span> offers additional hooks |
| with unique features: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| When the destructor of the hook is called, the hook checks if the node |
| is inserted in a container. If so, the hook removes the node from the |
| container. |
| </li> |
| <li class="listitem"> |
| The hook has a member function called <code class="computeroutput"><span class="identifier">unlink</span><span class="special">()</span></code> that can be used to unlink the node |
| from the container at any time, without having any reference to the container, |
| if the user wants to do so. |
| </li> |
| </ul></div> |
| <p> |
| These hooks have exactly the same size overhead as their analog non auto-unlinking |
| hooks, but they have a restriction: they can only be used with <a class="link" href="presenting_containers.html" title="Presenting Boost.Intrusive containers">non-constant |
| time containers</a>. There is a reason for this: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| Auto-unlink hooks don't store any reference to the container where they |
| are inserted. |
| </li> |
| <li class="listitem"> |
| Only containers with non constant-time <code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code> allow removing an object from the container |
| without referring to the container. |
| </li> |
| </ul></div> |
| <p> |
| This auto-unlink feature is useful in certain applications but it must be |
| used <span class="bold"><strong>very carefully</strong></span>: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| If several threads are using the same container the destructor of the |
| auto-unlink hook will be called without any thread synchronization so |
| removing the object is thread-unsafe. |
| </li> |
| <li class="listitem"> |
| Container contents change silently without modifying the container directly. |
| This can lead to surprising effects. |
| </li> |
| </ul></div> |
| <p> |
| These auto-unlink hooks have also safe-mode properties: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| Hooks' constructors put the hook in a well-known default state. |
| </li> |
| <li class="listitem"> |
| Every time an object is inserted in the intrusive container, the container |
| checks if the hook is in the well-known default state. If not, an assertion |
| is raised. |
| </li> |
| <li class="listitem"> |
| Every time an object is erased from an intrusive container, the container |
| puts the erased object in the well-known default state. |
| </li> |
| </ul></div> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="intrusive.auto_unlink_hooks.auto_unlink_hooks_example"></a><a class="link" href="auto_unlink_hooks.html#intrusive.auto_unlink_hooks.auto_unlink_hooks_example" title="Auto-unlink hook example">Auto-unlink |
| hook example</a> |
| </h3></div></div></div> |
| <p> |
| Let's see an example of an auto-unlink hook: |
| </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="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cassert</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="keyword">typedef</span> <span class="identifier">list_base_hook</span><span class="special"><</span><span class="identifier">link_mode</span><span class="special"><</span><span class="identifier">auto_unlink</span><span class="special">></span> <span class="special">></span> <span class="identifier">auto_unlink_hook</span><span class="special">;</span> |
| |
| <span class="keyword">class</span> <span class="identifier">MyClass</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">auto_unlink_hook</span> |
| <span class="comment">//This hook removes the node in the destructor |
| </span><span class="special">{</span> |
| <span class="keyword">int</span> <span class="identifier">int_</span><span class="special">;</span> |
| |
| <span class="keyword">public</span><span class="special">:</span> |
| <span class="identifier">MyClass</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="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">void</span> <span class="identifier">unlink</span><span class="special">()</span> <span class="special">{</span> <span class="identifier">auto_unlink_hook</span><span class="special">::</span><span class="identifier">unlink</span><span class="special">();</span> <span class="special">}</span> |
| <span class="keyword">bool</span> <span class="identifier">is_linked</span><span class="special">()</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">auto_unlink_hook</span><span class="special">::</span><span class="identifier">is_linked</span><span class="special">();</span> <span class="special">}</span> |
| <span class="special">};</span> |
| |
| <span class="comment">//Define a list that will store values using the base hook |
| </span><span class="comment">//The list can't have constant-time size! |
| </span><span class="keyword">typedef</span> <span class="identifier">list</span><span class="special"><</span> <span class="identifier">MyClass</span><span class="special">,</span> <span class="identifier">constant_time_size</span><span class="special"><</span><span class="keyword">false</span><span class="special">></span> <span class="special">></span> <span class="identifier">List</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="comment">//Create the list |
| </span> <span class="identifier">List</span> <span class="identifier">l</span><span class="special">;</span> |
| <span class="special">{</span> |
| <span class="comment">//Create myclass and check it's linked |
| </span> <span class="identifier">MyClass</span> <span class="identifier">myclass</span><span class="special">;</span> |
| <span class="identifier">assert</span><span class="special">(</span><span class="identifier">myclass</span><span class="special">.</span><span class="identifier">is_linked</span><span class="special">()</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span> |
| |
| <span class="comment">//Insert the object |
| </span> <span class="identifier">l</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">myclass</span><span class="special">);</span> |
| |
| <span class="comment">//Check that we have inserted the object |
| </span> <span class="identifier">assert</span><span class="special">(</span><span class="identifier">l</span><span class="special">.</span><span class="identifier">empty</span><span class="special">()</span> <span class="special">==</span> <span class="keyword">false</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">front</span><span class="special">()</span> <span class="special">==</span> <span class="special">&</span><span class="identifier">myclass</span><span class="special">);</span> |
| <span class="identifier">assert</span><span class="special">(</span><span class="identifier">myclass</span><span class="special">.</span><span class="identifier">is_linked</span><span class="special">()</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span> |
| |
| <span class="comment">//Now myclass' destructor will unlink it |
| </span> <span class="comment">//automatically |
| </span> <span class="special">}</span> |
| |
| <span class="comment">//Check auto-unlink has been executed |
| </span> <span class="identifier">assert</span><span class="special">(</span><span class="identifier">l</span><span class="special">.</span><span class="identifier">empty</span><span class="special">()</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span> |
| |
| <span class="special">{</span> |
| <span class="comment">//Now test the unlink() function |
| </span> |
| <span class="comment">//Create myclass and check it's linked |
| </span> <span class="identifier">MyClass</span> <span class="identifier">myclass</span><span class="special">;</span> |
| <span class="identifier">assert</span><span class="special">(</span><span class="identifier">myclass</span><span class="special">.</span><span class="identifier">is_linked</span><span class="special">()</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span> |
| |
| <span class="comment">//Insert the object |
| </span> <span class="identifier">l</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">myclass</span><span class="special">);</span> |
| |
| <span class="comment">//Check that we have inserted the object |
| </span> <span class="identifier">assert</span><span class="special">(</span><span class="identifier">l</span><span class="special">.</span><span class="identifier">empty</span><span class="special">()</span> <span class="special">==</span> <span class="keyword">false</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">front</span><span class="special">()</span> <span class="special">==</span> <span class="special">&</span><span class="identifier">myclass</span><span class="special">);</span> |
| <span class="identifier">assert</span><span class="special">(</span><span class="identifier">myclass</span><span class="special">.</span><span class="identifier">is_linked</span><span class="special">()</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span> |
| |
| <span class="comment">//Now unlink the node |
| </span> <span class="identifier">myclass</span><span class="special">.</span><span class="identifier">unlink</span><span class="special">();</span> |
| |
| <span class="comment">//Check auto-unlink has been executed |
| </span> <span class="identifier">assert</span><span class="special">(</span><span class="identifier">l</span><span class="special">.</span><span class="identifier">empty</span><span class="special">()</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</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> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="intrusive.auto_unlink_hooks.auto_unlink_and_constant_time"></a><a class="link" href="auto_unlink_hooks.html#intrusive.auto_unlink_hooks.auto_unlink_and_constant_time" title="Auto-unlink hooks and containers with constant-time size()">Auto-unlink |
| hooks and containers with constant-time <code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code></a> |
| </h3></div></div></div> |
| <p> |
| As explained, <span class="bold"><strong>Boost.Intrusive</strong></span> auto-unlink |
| hooks are incompatible with containers that have constant-time <code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code>, |
| so if you try to define such container with an auto-unlink hook's value_traits, |
| you will get a static assertion: |
| </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="identifier">boost</span><span class="special">::</span><span class="identifier">intrusive</span><span class="special">;</span> |
| |
| <span class="keyword">struct</span> <span class="identifier">MyTag</span><span class="special">;</span> |
| |
| <span class="keyword">class</span> <span class="identifier">MyClass</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">list_base_hook</span><span class="special"><</span> <span class="identifier">link_mode</span><span class="special"><</span><span class="identifier">auto_unlink</span><span class="special">></span> <span class="special">></span> |
| <span class="special">{/**/};</span> |
| |
| <span class="identifier">list</span> <span class="special"><</span><span class="identifier">MyClass</span><span class="special">,</span> <span class="identifier">constant_time_size</span><span class="special"><</span><span class="keyword">true</span><span class="special">></span> <span class="special">></span> <span class="identifier">bad_list</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">bad_list</span> <span class="identifier">list</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> |
| leads to an error similar to: |
| </p> |
| <pre class="programlisting">error : use of undefined type 'boost::STATIC_ASSERTION_FAILURE<false>' |
| </pre> |
| <p> |
| Pointing to code like this: |
| </p> |
| <pre class="programlisting"><span class="comment">//Constant-time size is incompatible with auto-unlink hooks! |
| </span><span class="identifier">BOOST_STATIC_ASSERT</span><span class="special">(!(</span><span class="identifier">constant_time_size</span> <span class="special">&&</span> <span class="special">((</span><span class="keyword">int</span><span class="special">)</span><span class="identifier">value_traits</span><span class="special">::</span><span class="identifier">link_mode</span> <span class="special">==</span> <span class="special">(</span><span class="keyword">int</span><span class="special">)</span><span class="identifier">auto_unlink</span><span class="special">)));</span> |
| </pre> |
| <p> |
| This way, there is no way to compile a program if you try to use auto-unlink |
| hooks in constant-time size containers. |
| </p> |
| </div> |
| </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> |
| </div></td> |
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