1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<head> 4<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 5<title>Intrusive and non-intrusive containers</title> 6<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> 7<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 8<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> 9<link rel="up" href="../intrusive.html" title="Chapter 19. Boost.Intrusive"> 10<link rel="prev" href="../intrusive.html" title="Chapter 19. Boost.Intrusive"> 11<link rel="next" href="usage.html" title="How to use Boost.Intrusive"> 12</head> 13<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 14<table cellpadding="2" width="100%"><tr> 15<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> 16<td align="center"><a href="../../../index.html">Home</a></td> 17<td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> 18<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 19<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 20<td align="center"><a href="../../../more/index.htm">More</a></td> 21</tr></table> 22<hr> 23<div class="spirit-nav"> 24<a accesskey="p" href="../intrusive.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="usage.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 25</div> 26<div class="section"> 27<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 28<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 29 containers</a> 30</h2></div></div></div> 31<div class="toc"><dl class="toc"> 32<dt><span class="section"><a href="intrusive_vs_nontrusive.html#intrusive.intrusive_vs_nontrusive.differences_intrusive_vs_nontrusive">Differences 33 between intrusive and non-intrusive containers</a></span></dt> 34<dt><span class="section"><a href="intrusive_vs_nontrusive.html#intrusive.intrusive_vs_nontrusive.properties_of_intrusive">Properties 35 of Boost.Intrusive containers</a></span></dt> 36</dl></div> 37<div class="section"> 38<div class="titlepage"><div><div><h3 class="title"> 39<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 40 between intrusive and non-intrusive containers</a> 41</h3></div></div></div> 42<p> 43 The main difference between intrusive containers and non-intrusive containers 44 is that in C++ non-intrusive containers store <span class="bold"><strong>copies</strong></span> 45 of values passed by the user. Containers use the <code class="computeroutput"><span class="identifier">Allocator</span></code> 46 template parameter to allocate the stored values: 47 </p> 48<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">list</span><span class="special">></span> 49<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> 50 51<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 52<span class="special">{</span> 53 <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> 54 55 <span class="identifier">MyClass</span> <span class="identifier">myclass</span><span class="special">(...);</span> 56 <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> 57 58 <span class="comment">//The stored object is different from the original object</span> 59 <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> 60 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 61<span class="special">}</span> 62</pre> 63<p> 64 To store the newly allocated copy of <code class="computeroutput"><span class="identifier">myclass</span></code>, 65 the container needs additional data: <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span></code> 66 usually allocates nodes that contain pointers to the next and previous node 67 and the value itself. Something similar to: 68 </p> 69<pre class="programlisting"><span class="comment">//A possible implementation of a std::list<MyClass> node</span> 70<span class="keyword">class</span> <span class="identifier">list_node</span> 71<span class="special">{</span> 72 <span class="identifier">list_node</span> <span class="special">*</span><span class="identifier">next</span><span class="special">;</span> 73 <span class="identifier">list_node</span> <span class="special">*</span><span class="identifier">previous</span><span class="special">;</span> 74 <span class="identifier">MyClass</span> <span class="identifier">value</span><span class="special">;</span> 75<span class="special">};</span> 76</pre> 77<p> 78 On the other hand, an intrusive container does not store copies of passed 79 objects, but it stores the objects themselves. The additional data needed 80 to insert the object in the container must be provided by the object itself. 81 For example, to insert <code class="computeroutput"><span class="identifier">MyClass</span></code> 82 in an intrusive container that implements a linked list, <code class="computeroutput"><span class="identifier">MyClass</span></code> 83 must contain the needed <span class="emphasis"><em>next</em></span> and <span class="emphasis"><em>previous</em></span> 84 pointers: 85 </p> 86<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">MyClass</span> 87<span class="special">{</span> 88 <span class="identifier">MyClass</span> <span class="special">*</span><span class="identifier">next</span><span class="special">;</span> 89 <span class="identifier">MyClass</span> <span class="special">*</span><span class="identifier">previous</span><span class="special">;</span> 90 <span class="comment">//Other members...</span> 91<span class="special">};</span> 92 93<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 94<span class="special">{</span> 95 <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> 96 97 <span class="identifier">MyClass</span> <span class="identifier">myclass</span><span class="special">;</span> 98 <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> 99 100 <span class="comment">//"myclass" object is stored in the list</span> 101 <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> 102 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 103<span class="special">}</span> 104</pre> 105<p> 106 As we can see, knowing which additional data the class should contain is 107 not an easy task. <span class="bold"><strong>Boost.Intrusive</strong></span> offers 108 several intrusive containers and an easy way to make user classes compatible 109 with those containers. 110 </p> 111</div> 112<div class="section"> 113<div class="titlepage"><div><div><h3 class="title"> 114<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 115 of Boost.Intrusive containers</a> 116</h3></div></div></div> 117<p> 118 Semantically, a <span class="bold"><strong>Boost.Intrusive</strong></span> container 119 is similar to a STL container holding pointers to objects. That is, if you 120 have an intrusive list holding objects of type <code class="computeroutput"><span class="identifier">T</span></code>, 121 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> 122 would allow you to do quite the same operations (maintaining and navigating 123 a set of objects of type T and types derived from it). 124 </p> 125<p> 126 A non-intrusive container has some limitations: 127 </p> 128<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 129<li class="listitem"> 130 An object can only belong to one container: If you want to share an object 131 between two containers, you either have to store multiple copies of those 132 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>. 133 </li> 134<li class="listitem"> 135 The use of dynamic allocation to create copies of passed values can be 136 a performance and size bottleneck in some applications. Normally, dynamic 137 allocation imposes a size overhead for each allocation to store bookkeeping 138 information and a synchronization to protected concurrent allocation 139 from different threads. 140 </li> 141<li class="listitem"> 142 Before C++11, only copies of objects could be stored in non-intrusive 143 containers. Still copy or move constructors and copy or move assignment 144 operators are required and non-copyable and non-movable objects can't 145 be stored in some containers. In any case, <span class="bold"><strong>new</strong></span> 146 objects have to be created inside the container using constructors and 147 the same object can't be shared between two containers. 148 </li> 149<li class="listitem"> 150 It's not possible to store a derived object in a STL-container while 151 retaining its original type. 152 </li> 153</ul></div> 154<p> 155 Intrusive containers have some important advantages: 156 </p> 157<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 158<li class="listitem"> 159 Operating with intrusive containers doesn't invoke any memory management 160 at all. The time and size overhead associated with dynamic memory can 161 be minimized. 162 </li> 163<li class="listitem"> 164 The same object can be inserted in more than one container at the same 165 time with a tiny overhead in the object size. 166 </li> 167<li class="listitem"> 168 Iterating an Intrusive container needs less memory accesses than the 169 semantically equivalent container of pointers: iteration is faster. 170 </li> 171<li class="listitem"> 172 Intrusive containers offer better exception guarantees than non-intrusive 173 containers. In some situations intrusive containers offer a no-throw 174 guarantee that can't be achieved with non-intrusive containers. 175 </li> 176<li class="listitem"> 177 The computation of an iterator to an element from a pointer or reference 178 to that element is a constant time operation (computing the position 179 of <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span></code> 180 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> 181 has linear complexity). 182 </li> 183<li class="listitem"> 184 Intrusive containers offer predictability when inserting and erasing 185 objects since no memory management is done with intrusive containers. 186 Memory management usually is not a predictable operation so complexity 187 guarantees from non-intrusive containers are looser than the guarantees 188 offered by intrusive containers. 189 </li> 190</ul></div> 191<p> 192 Intrusive containers have also downsides: 193 </p> 194<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 195<li class="listitem"> 196 Each type stored in an intrusive container needs additional memory holding 197 the maintenance information needed by the container. Hence, whenever 198 a certain type will be stored in an intrusive container <span class="bold"><strong>you 199 have to change the definition of that type</strong></span> appropriately. 200 Although this task is easy with <span class="bold"><strong>Boost.Intrusive</strong></span>, 201 touching the definition of a type is sometimes a crucial issue. 202 </li> 203<li class="listitem"> 204 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 205 in the container</strong></span>. Objects don't need copy-constructors or 206 assignment operators to be stored in intrusive containers. But you have 207 to take care of possible side effects, whenever you change the contents 208 of an object (this is especially important for associative containers). 209 </li> 210<li class="listitem"> 211 The user <span class="bold"><strong>has to manage the lifetime of inserted 212 objects</strong></span> independently from the containers. 213 </li> 214<li class="listitem"> 215 Again you have to be <span class="bold"><strong>careful</strong></span>: in contrast 216 to STL containers <span class="bold"><strong>it's easy to render an iterator 217 invalid</strong></span> without touching the intrusive container directly, 218 because the object can be disposed before is erased from the container. 219 </li> 220<li class="listitem"> 221 <span class="bold"><strong>Boost.Intrusive</strong></span> containers are <span class="bold"><strong>non-copyable and non-assignable</strong></span>. Since intrusive 222 containers don't have allocation capabilities, these operations make 223 no sense. However, swapping can be used to implement move capabilities. 224 To ease the implementation of copy constructors and assignment operators 225 of classes storing <span class="bold"><strong>Boost.Intrusive</strong></span> containers, 226 <span class="bold"><strong>Boost.Intrusive</strong></span> offers special cloning 227 functions. See <a class="link" href="clone_from.html" title="Cloning Boost.Intrusive containers">Cloning Boost.Intrusive 228 containers</a> section for more information. 229 </li> 230<li class="listitem"> 231 Analyzing the thread safety of a program that uses containers is harder 232 with intrusive containers, because the container might be modified indirectly 233 without an explicit call to a container member. 234 </li> 235</ul></div> 236<div class="table"> 237<a name="intrusive.intrusive_vs_nontrusive.properties_of_intrusive.summary_of_intrusive_containers_"></a><p class="title"><b>Table 19.1. Summary of intrusive containers advantages and disadvantages</b></p> 238<div class="table-contents"><table class="table" summary="Summary of intrusive containers advantages and disadvantages"> 239<colgroup> 240<col> 241<col> 242<col> 243</colgroup> 244<thead><tr> 245<th> 246 <p> 247 Issue 248 </p> 249 </th> 250<th> 251 <p> 252 Intrusive 253 </p> 254 </th> 255<th> 256 <p> 257 Non-intrusive 258 </p> 259 </th> 260</tr></thead> 261<tbody> 262<tr> 263<td> 264 <p> 265 Memory management 266 </p> 267 </td> 268<td> 269 <p> 270 External 271 </p> 272 </td> 273<td> 274 <p> 275 Internal through allocator 276 </p> 277 </td> 278</tr> 279<tr> 280<td> 281 <p> 282 Insertion/Erasure time 283 </p> 284 </td> 285<td> 286 <p> 287 Faster 288 </p> 289 </td> 290<td> 291 <p> 292 Slower 293 </p> 294 </td> 295</tr> 296<tr> 297<td> 298 <p> 299 Memory locality 300 </p> 301 </td> 302<td> 303 <p> 304 Better 305 </p> 306 </td> 307<td> 308 <p> 309 Worse 310 </p> 311 </td> 312</tr> 313<tr> 314<td> 315 <p> 316 Can insert the same object in more than one container 317 </p> 318 </td> 319<td> 320 <p> 321 Yes 322 </p> 323 </td> 324<td> 325 <p> 326 No 327 </p> 328 </td> 329</tr> 330<tr> 331<td> 332 <p> 333 Exception guarantees 334 </p> 335 </td> 336<td> 337 <p> 338 Better 339 </p> 340 </td> 341<td> 342 <p> 343 Worse 344 </p> 345 </td> 346</tr> 347<tr> 348<td> 349 <p> 350 Computation of iterator from value 351 </p> 352 </td> 353<td> 354 <p> 355 Constant 356 </p> 357 </td> 358<td> 359 <p> 360 Non-constant 361 </p> 362 </td> 363</tr> 364<tr> 365<td> 366 <p> 367 Insertion/erasure predictability 368 </p> 369 </td> 370<td> 371 <p> 372 High 373 </p> 374 </td> 375<td> 376 <p> 377 Low 378 </p> 379 </td> 380</tr> 381<tr> 382<td> 383 <p> 384 Memory use 385 </p> 386 </td> 387<td> 388 <p> 389 Minimal 390 </p> 391 </td> 392<td> 393 <p> 394 More than minimal 395 </p> 396 </td> 397</tr> 398<tr> 399<td> 400 <p> 401 Insert objects by value retaining polymorphic behavior 402 </p> 403 </td> 404<td> 405 <p> 406 Yes 407 </p> 408 </td> 409<td> 410 <p> 411 No (slicing) 412 </p> 413 </td> 414</tr> 415<tr> 416<td> 417 <p> 418 User must modify the definition of the values to insert 419 </p> 420 </td> 421<td> 422 <p> 423 Yes 424 </p> 425 </td> 426<td> 427 <p> 428 No 429 </p> 430 </td> 431</tr> 432<tr> 433<td> 434 <p> 435 Containers are copyable 436 </p> 437 </td> 438<td> 439 <p> 440 No 441 </p> 442 </td> 443<td> 444 <p> 445 Yes 446 </p> 447 </td> 448</tr> 449<tr> 450<td> 451 <p> 452 Inserted object's lifetime managed by 453 </p> 454 </td> 455<td> 456 <p> 457 User (more complex) 458 </p> 459 </td> 460<td> 461 <p> 462 Container (less complex) 463 </p> 464 </td> 465</tr> 466<tr> 467<td> 468 <p> 469 Container invariants can be broken without using the container 470 </p> 471 </td> 472<td> 473 <p> 474 Easier 475 </p> 476 </td> 477<td> 478 <p> 479 Harder (only with containers of pointers) 480 </p> 481 </td> 482</tr> 483<tr> 484<td> 485 <p> 486 Thread-safety analysis 487 </p> 488 </td> 489<td> 490 <p> 491 Harder 492 </p> 493 </td> 494<td> 495 <p> 496 Easier 497 </p> 498 </td> 499</tr> 500</tbody> 501</table></div> 502</div> 503<br class="table-break"><p> 504 For a performance comparison between Intrusive and Non-intrusive containers 505 see <a class="link" href="performance.html" title="Performance">Performance</a> section. 506 </p> 507</div> 508</div> 509<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 510<td align="left"></td> 511<td align="right"><div class="copyright-footer">Copyright © 2005 Olaf Krzikalla<br>Copyright © 2006-2015 Ion Gaztanaga<p> 512 Distributed under the Boost Software License, Version 1.0. (See accompanying 513 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>) 514 </p> 515</div></td> 516</tr></table> 517<hr> 518<div class="spirit-nav"> 519<a accesskey="p" href="../intrusive.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="usage.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 520</div> 521</body> 522</html> 523
