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| <div class="section"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="intrusive.intrusive_vs_nontrusive"></a><a class="link" href="intrusive_vs_nontrusive.html" title="Intrusive and non-intrusive containers">Intrusive and non-intrusive |
| containers</a> |
| </h2></div></div></div> |
| <div class="toc"><dl> |
| <dt><span class="section"><a href="intrusive_vs_nontrusive.html#intrusive.intrusive_vs_nontrusive.differences_intrusive_vs_nontrusive">Differences |
| between intrusive and non-intrusive containers</a></span></dt> |
| <dt><span class="section"><a href="intrusive_vs_nontrusive.html#intrusive.intrusive_vs_nontrusive.properties_of_intrusive">Properties |
| of Boost.Intrusive containers</a></span></dt> |
| </dl></div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="intrusive.intrusive_vs_nontrusive.differences_intrusive_vs_nontrusive"></a><a class="link" href="intrusive_vs_nontrusive.html#intrusive.intrusive_vs_nontrusive.differences_intrusive_vs_nontrusive" title="Differences between intrusive and non-intrusive containers">Differences |
| between intrusive and non-intrusive containers</a> |
| </h3></div></div></div> |
| <p> |
| The main difference between intrusive containers and non-intrusive containers |
| is that in C++ non-intrusive containers store <span class="bold"><strong>copies</strong></span> |
| of values passed by the user. Containers use the <code class="computeroutput"><span class="identifier">Allocator</span></code> |
| template parameter to allocate the stored values: |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">list</span><span class="special">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">assert</span><span class="special">.</span><span class="identifier">h</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">std</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">myclass_list</span><span class="special">;</span> |
| |
| <span class="identifier">MyClass</span> <span class="identifier">myclass</span><span class="special">(...);</span> |
| <span class="identifier">myclass_list</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">//The stored object is different from the original object |
| </span> <span class="identifier">assert</span><span class="special">(&</span><span class="identifier">myclass</span> <span class="special">!=</span> <span class="special">&</span><span class="identifier">myclass_list</span><span class="special">.</span><span class="identifier">front</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> |
| To store the newly allocated copy of <code class="computeroutput"><span class="identifier">myclass</span></code>, |
| the container needs additional data: <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span></code> |
| usually allocates nodes that contain pointers to the next and previous node |
| and the value itself. Something similar to: |
| </p> |
| <pre class="programlisting"><span class="comment">//A possible implementation of a std::list<MyClass> node |
| </span><span class="keyword">class</span> <span class="identifier">list_node</span> |
| <span class="special">{</span> |
| <span class="identifier">list_node</span> <span class="special">*</span><span class="identifier">next</span><span class="special">;</span> |
| <span class="identifier">list_node</span> <span class="special">*</span><span class="identifier">previous</span><span class="special">;</span> |
| <span class="identifier">MyClass</span> <span class="identifier">value</span><span class="special">;</span> |
| <span class="special">};</span> |
| </pre> |
| <p> |
| On the other hand, an intrusive container does not store copies of passed |
| objects, but it stores the objects themselves. The additional data needed |
| to insert the object in the container must be provided by the object itself. |
| For example, to insert <code class="computeroutput"><span class="identifier">MyClass</span></code> |
| in an intrusive container that implements a linked list, <code class="computeroutput"><span class="identifier">MyClass</span></code> |
| must contain the needed <span class="emphasis"><em>next</em></span> and <span class="emphasis"><em>previous</em></span> |
| pointers: |
| </p> |
| <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">MyClass</span> |
| <span class="special">{</span> |
| <span class="identifier">MyClass</span> <span class="special">*</span><span class="identifier">next</span><span class="special">;</span> |
| <span class="identifier">MyClass</span> <span class="special">*</span><span class="identifier">previous</span><span class="special">;</span> |
| <span class="comment">//Other members... |
| </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">acme_intrusive_list</span><span class="special"><</span><span class="identifier">MyClass</span><span class="special">></span> <span class="identifier">list</span><span class="special">;</span> |
| |
| <span class="identifier">MyClass</span> <span class="identifier">myclass</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="identifier">myclass</span><span class="special">);</span> |
| |
| <span class="comment">//"myclass" object is stored in the list |
| </span> <span class="identifier">assert</span><span class="special">(&</span><span class="identifier">myclass</span> <span class="special">==</span> <span class="special">&</span><span class="identifier">list</span><span class="special">.</span><span class="identifier">front</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> |
| As we can see, knowing which additional data the class should contain is |
| not an easy task. <span class="bold"><strong>Boost.Intrusive</strong></span> offers |
| several intrusive containers and an easy way to make user classes compatible |
| with those containers. |
| </p> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="intrusive.intrusive_vs_nontrusive.properties_of_intrusive"></a><a class="link" href="intrusive_vs_nontrusive.html#intrusive.intrusive_vs_nontrusive.properties_of_intrusive" title="Properties of Boost.Intrusive containers">Properties |
| of Boost.Intrusive containers</a> |
| </h3></div></div></div> |
| <p> |
| Semantically, a <span class="bold"><strong>Boost.Intrusive</strong></span> container |
| is similar to a STL container holding pointers to objects. That is, if you |
| have an intrusive list holding objects of type <code class="computeroutput"><span class="identifier">T</span></code>, |
| then <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="identifier">T</span><span class="special">*></span></code> |
| would allow you to do quite the same operations (maintaining and navigating |
| a set of objects of type T and types derived from it). |
| </p> |
| <p> |
| A non-intrusive container has some limitations: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| An object can only belong to one container: If you want to share an object |
| between two containers, you either have to store multiple copies of those |
| objects or you need to use containers of pointers: <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>. |
| </li> |
| <li class="listitem"> |
| The use of dynamic allocation to create copies of passed values can be |
| a performance and size bottleneck in some applications. Normally, dynamic |
| allocation imposes a size overhead for each allocation to store bookkeeping |
| information and a synchronization to protected concurrent allocation |
| from different threads. |
| </li> |
| <li class="listitem"> |
| Only copies of objects are stored in non-intrusive containers. Hence |
| copy or move constructors and copy or move assignment operators are required. |
| Non-copyable and non-movable objects can't be stored in non-intrusive |
| containers. |
| </li> |
| <li class="listitem"> |
| It's not possible to store a derived object in a STL-container while |
| retaining its original type. |
| </li> |
| </ul></div> |
| <p> |
| Intrusive containers have some important advantages: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| Operating with intrusive containers doesn't invoke any memory management |
| at all. The time and size overhead associated with dynamic memory can |
| be minimized. |
| </li> |
| <li class="listitem"> |
| Iterating an Intrusive container needs less memory accesses than the |
| semantically equivalent container of pointers: iteration is faster. |
| </li> |
| <li class="listitem"> |
| Intrusive containers offer better exception guarantees than non-intrusive |
| containers. In some situations intrusive containers offer a no-throw |
| guarantee that can't be achieved with non-intrusive containers. |
| </li> |
| <li class="listitem"> |
| The computation of an iterator to an element from a pointer or reference |
| to that element is a constant time operation (computing the position |
| of <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span></code> |
| in a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="identifier">T</span><span class="special">*></span></code> |
| has linear complexity). |
| </li> |
| <li class="listitem"> |
| Intrusive containers offer predictability when inserting and erasing |
| objects since no memory management is done with intrusive containers. |
| Memory management usually is not a predictable operation so complexity |
| guarantees from non-intrusive containers are looser than the guarantees |
| offered by intrusive containers. |
| </li> |
| </ul></div> |
| <p> |
| Intrusive containers have also downsides: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| Each type stored in an intrusive container needs additional memory holding |
| the maintenance information needed by the container. Hence, whenever |
| a certain type will be stored in an intrusive container <span class="bold"><strong>you |
| have to change the definition of that type</strong></span> appropriately. |
| Although this task is easy with <span class="bold"><strong>Boost.Intrusive</strong></span>, |
| touching the definition of a type is sometimes a crucial issue. |
| </li> |
| <li class="listitem"> |
| In intrusive containers you don't store a copy of an object, <span class="bold"><strong>but rather the original object is linked with other objects |
| in the container</strong></span>. Objects don't need copy-constructors or |
| assignment operators to be stored in intrusive containers. But you have |
| to take care of possible side effects, whenever you change the contents |
| of an object (this is especially important for associative containers). |
| </li> |
| <li class="listitem"> |
| The user <span class="bold"><strong>has to manage the lifetime of inserted |
| objects</strong></span> independently from the containers. |
| </li> |
| <li class="listitem"> |
| Again you have to be <span class="bold"><strong>careful</strong></span>: in contrast |
| to STL containers <span class="bold"><strong>it's easy to render an iterator |
| invalid</strong></span> without touching the intrusive container directly, |
| because the object can be disposed before is erased from the container. |
| </li> |
| <li class="listitem"> |
| <span class="bold"><strong>Boost.Intrusive</strong></span> containers are <span class="bold"><strong>non-copyable and non-assignable</strong></span>. Since intrusive |
| containers don't have allocation capabilities, these operations make |
| no sense. However, swapping can be used to implement move capabilities. |
| To ease the implementation of copy constructors and assignment operators |
| of classes storing <span class="bold"><strong>Boost.Intrusive</strong></span> containers, |
| <span class="bold"><strong>Boost.Intrusive</strong></span> offers special cloning |
| functions. See <a class="link" href="clone_from.html" title="Cloning Boost.Intrusive containers">Cloning Boost.Intrusive |
| containers</a> section for more information. |
| </li> |
| <li class="listitem"> |
| Analyzing the thread safety of a program that uses containers is harder |
| with intrusive containers, because the container might be modified indirectly |
| without an explicit call to a container member. |
| </li> |
| </ul></div> |
| <div class="table"> |
| <a name="id1564362"></a><p class="title"><b>Table 10.1. Summary of intrusive containers advantages and disadvantages</b></p> |
| <div class="table-contents"><table class="table" summary="Summary of intrusive containers advantages and disadvantages"> |
| <colgroup> |
| <col> |
| <col> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th> |
| <p> |
| Issue |
| </p> |
| </th> |
| <th> |
| <p> |
| Intrusive |
| </p> |
| </th> |
| <th> |
| <p> |
| Non-intrusive |
| </p> |
| </th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td> |
| <p> |
| Memory management |
| </p> |
| </td> |
| <td> |
| <p> |
| External |
| </p> |
| </td> |
| <td> |
| <p> |
| Internal through allocator |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| Insertion/Erasure time |
| </p> |
| </td> |
| <td> |
| <p> |
| Faster |
| </p> |
| </td> |
| <td> |
| <p> |
| Slower |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| Memory locality |
| </p> |
| </td> |
| <td> |
| <p> |
| Better |
| </p> |
| </td> |
| <td> |
| <p> |
| Worse |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| Can hold non-copyable and non-movable objects by value |
| </p> |
| </td> |
| <td> |
| <p> |
| Yes |
| </p> |
| </td> |
| <td> |
| <p> |
| No |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| Exception guarantees |
| </p> |
| </td> |
| <td> |
| <p> |
| Better |
| </p> |
| </td> |
| <td> |
| <p> |
| Worse |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| Computation of iterator from value |
| </p> |
| </td> |
| <td> |
| <p> |
| Constant |
| </p> |
| </td> |
| <td> |
| <p> |
| Non-constant |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| Insertion/erasure predictability |
| </p> |
| </td> |
| <td> |
| <p> |
| High |
| </p> |
| </td> |
| <td> |
| <p> |
| Low |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| Memory use |
| </p> |
| </td> |
| <td> |
| <p> |
| Minimal |
| </p> |
| </td> |
| <td> |
| <p> |
| More than minimal |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| Insert objects by value retaining polymorphic behavior |
| </p> |
| </td> |
| <td> |
| <p> |
| Yes |
| </p> |
| </td> |
| <td> |
| <p> |
| No (slicing) |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| User must modify the definition of the values to insert |
| </p> |
| </td> |
| <td> |
| <p> |
| Yes |
| </p> |
| </td> |
| <td> |
| <p> |
| No |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| Containers are copyable |
| </p> |
| </td> |
| <td> |
| <p> |
| No |
| </p> |
| </td> |
| <td> |
| <p> |
| Yes |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| Inserted object's lifetime managed by |
| </p> |
| </td> |
| <td> |
| <p> |
| User (more complex) |
| </p> |
| </td> |
| <td> |
| <p> |
| Container (less complex) |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| Container invariants can be broken without using the container |
| </p> |
| </td> |
| <td> |
| <p> |
| Easier |
| </p> |
| </td> |
| <td> |
| <p> |
| Harder (only with containers of pointers) |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| Thread-safety analysis |
| </p> |
| </td> |
| <td> |
| <p> |
| Harder |
| </p> |
| </td> |
| <td> |
| <p> |
| Easier |
| </p> |
| </td> |
| </tr> |
| </tbody> |
| </table></div> |
| </div> |
| <br class="table-break"><p> |
| For a performance comparison between Intrusive and Non-intrusive containers |
| see <a class="link" href="performance.html" title="Performance">Performance</a> section. |
| </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> |
| </tr></table> |
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