<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Auto-unlink hooks</title> <link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.75.2"> <link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> <link rel="up" href="../intrusive.html" title="Chapter 10. Boost.Intrusive"> <link rel="prev" href="safe_hook.html" title="Safe hooks"> <link rel="next" href="slist.html" title="Intrusive singly linked list: slist"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> <td align="center"><a href="../../../index.html">Home</a></td> <td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="safe_hook.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../intrusive.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="slist.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> <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> <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> <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> <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="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> <p> </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, 2006-2009 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> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="safe_hook.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../intrusive.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="slist.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>