ceph/src/boost/libs/ptr_container/doc/guidelines.html

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 287 </style>
 288 </head>
 289 <body>
 290 <div class="document" id="boost-pointer-container-library">
 291 <h1 class="title"><img alt="Boost" src="boost.png" /> Pointer Container Library</h1>
 292 <h2 class="subtitle" id="usage-guidelines">Usage Guidelines</h2>
 293 <div class="contents local topic">
 294 <ul class="simple">
 295 <li><a class="reference" href="#choosing-the-right-container" id="id8" name="id8">Choosing the right container</a></li>
 296 <li><a class="reference" href="#recommended-practice-for-object-oriented-programming" id="id9" name="id9">Recommended practice for Object-Oriented Programming</a><ul>
 297 <li><a class="reference" href="#make-base-classes-abstract-and-without-data" id="id10" name="id10">1. Make base classes abstract and without data</a></li>
 298 <li><a class="reference" href="#make-virtual-functions-private-and-provide-a-non-virtual-public-forwarding-function" id="id11" name="id11">2. Make virtual functions private and provide a non-virtual public forwarding function</a></li>
 299 <li><a class="reference" href="#derive-your-base-class-from-boost-noncopyable" id="id12" name="id12">3. Derive your base class from <tt class="docutils literal"><span class="pre">boost::noncopyable</span></tt></a></li>
 300 <li><a class="reference" href="#avoid-null-pointers-in-containers-if-possible" id="id13" name="id13">4. Avoid null-pointers in containers (if possible)</a></li>
 301 </ul>
 302 </li>
 303 </ul>
 304 </div>
 305 <div class="section">
 306 <h1><a class="toc-backref" href="#id8" id="choosing-the-right-container" name="choosing-the-right-container">Choosing the right container</a></h1>
 307 <p>The recommended usage pattern of the container classes is the same as
 308 for normal standard containers.</p>
 309 <p><tt class="docutils literal"><span class="pre">ptr_vector</span></tt>, <tt class="docutils literal"><span class="pre">ptr_list</span></tt> and <tt class="docutils literal"><span class="pre">ptr_deque</span></tt> offer the programmer different
 310 complexity tradeoffs and should be used accordingly.  <tt class="docutils literal"><span class="pre">ptr_vector</span></tt> is the
 311 type of sequence that should be used by default.  <tt class="docutils literal"><span class="pre">ptr_list</span></tt> should be used
 312 when there are frequent insertions and deletions from the middle of the
 313 sequence and if the container is fairly large (eg.  more than 100
 314 elements).  <tt class="docutils literal"><span class="pre">ptr_deque</span></tt> is the data structure of choice when most insertions
 315 and deletions take place at the beginning or at the end of the sequence.
 316 The special container <tt class="docutils literal"><span class="pre">ptr_array</span></tt> may be used when the size of the container is invariant
 317 and known at compile time.</p>
 318 <p>An associative container supports unique keys if it may contain at most
 319 one element for each key. Otherwise, it supports equivalent keys.
 320 <tt class="docutils literal"><span class="pre">ptr_set</span></tt> and <tt class="docutils literal"><span class="pre">ptr_map</span></tt> support unique keys.
 321 <tt class="docutils literal"><span class="pre">ptr_multiset</span></tt> and <tt class="docutils literal"><span class="pre">ptr_multimap</span></tt>
 322 support equivalent keys.</p>
 323 </div>
 324 <div class="section">
 325 <h1><a class="toc-backref" href="#id9" id="recommended-practice-for-object-oriented-programming" name="recommended-practice-for-object-oriented-programming">Recommended practice for Object-Oriented Programming</a></h1>
 326 <p>Idiomatic Object-Oriented Programming in C++ looks a bit different from
 327 the way it is done in other languages. This is partly because C++
 328 has both value and reference semantics, and partly because C++ is more flexible
 329 than other languages. Below is a list of recommendations that you are
 330 encouraged to follow:</p>
 331 <div class="section">
 332 <h2><a class="toc-backref" href="#id10" id="make-base-classes-abstract-and-without-data" name="make-base-classes-abstract-and-without-data">1. Make base classes abstract and without data</a></h2>
 333 <p>This has the following advantages:</p>
 334 <blockquote>
 335 <ol class="loweralpha simple">
 336 <li>It reduces <em>coupling</em> because you do not have to maintain or update state</li>
 337 </ol>
 338 <!--  -->
 339 <ol class="loweralpha simple" start="2">
 340 <li>It helps you to avoid <em>slicing</em></li>
 341 </ol>
 342 <!--  -->
 343 <ol class="loweralpha simple" start="3">
 344 <li>It ensures you <em>override</em> the right function</li>
 345 </ol>
 346 </blockquote>
 347 <p>You might also want to read the following articles:</p>
 348 <ul class="simple">
 349 <li>Kevlin Henney's <a class="reference" href="http://www.two-sdg.demon.co.uk/curbralan/papers/SixOfTheBest.pdf">Six of the best</a></li>
 350 </ul>
 351 <ul class="simple">
 352 <li>Jack Reeves' <a class="reference" href="http://www.ddj.com/documents/s=10011/q=1/cuj0602reeves/0602reeves.html">Multiple Inheritance Considered Useful</a></li>
 353 </ul>
 354 </div>
 355 <div class="section">
 356 <h2><a class="toc-backref" href="#id11" id="make-virtual-functions-private-and-provide-a-non-virtual-public-forwarding-function" name="make-virtual-functions-private-and-provide-a-non-virtual-public-forwarding-function">2. Make virtual functions private and provide a non-virtual public forwarding function</a></h2>
 357 <p>In code:</p>
 358 <pre class="literal-block">
 359 class Polymorphic
 360 {
 361 private:
 362     virtual int do_foo() = 0;
 363
 364 public:
 365     int foo()
 366     {
 367         return do_foo();
 368     }
 369     ...
 370 };
 371 </pre>
 372 <p>This has the following advantages:</p>
 373 <blockquote>
 374 <ol class="loweralpha simple">
 375 <li>It makes sure all calls to the virtual function always goes through one place in your code</li>
 376 </ol>
 377 <!--  -->
 378 <ol class="loweralpha simple" start="2">
 379 <li>It enables you to check preconditions and postconditions inside the forwarding function</li>
 380 </ol>
 381 </blockquote>
 382 <p>You might also want to read Herb Sutter's article <a class="reference" href="http://www.gotw.ca/publications/mill18.htm">Virtuality</a>.</p>
 383 </div>
 384 <div class="section">
 385 <h2><a class="toc-backref" href="#id12" id="derive-your-base-class-from-boost-noncopyable" name="derive-your-base-class-from-boost-noncopyable">3. Derive your base class from <tt class="docutils literal docutils literal"><span class="pre">boost::noncopyable</span></tt></a></h2>
 386 <p>Having an abstact base class prevents slicing when the base class is involved, but
 387 it does not prevent it for classes further down the hierarchy. This is where
 388 <a class="reference" href="http://www.boost.org/libs/utility/utility.htm#Class_noncopyable">boost::noncopyable</a> is handy to use:</p>
 389 <pre class="literal-block">
 390 class Polymorphic : boost::noncopyable
 391 {
 392   ...
 393 };
 394 </pre>
 395 </div>
 396 <div class="section">
 397 <h2><a class="toc-backref" href="#id13" id="avoid-null-pointers-in-containers-if-possible" name="avoid-null-pointers-in-containers-if-possible">4. Avoid null-pointers in containers (if possible)</a></h2>
 398 <p>By default the pointer containers do not allow you to store null-pointer in them.
 399 As you might know, this behavior can be changed explicitly with the use
 400 of <a class="reference" href="reference.html#class-nullable">boost::nullable</a>.</p>
 401 <p>The primary reason to avoid null-pointers
 402 is that you have to check for null-pointers every time the container is
 403 used. This extra checking is easy to forget, and it is somewhat contradictory to
 404 the spirit of OO where you replace special cases with dynamic dispatch.</p>
 405 <p>Often, however, you need to place some special object in the container because you
 406 do not have enough information to construct a full object. In that case
 407 you might be able to use the Null Object pattern which simply dictates that
 408 you implement virtual functions from the abstract base-class
 409 as empty functions or with dummy return values. This means that
 410 your OO-code still does not need to worry about null-pointers.</p>
 411 <p>You might want to read</p>
 412 <ul class="simple">
 413 <li>Kevlin Henney's <a class="reference" href="http://www.two-sdg.demon.co.uk/curbralan/papers/europlop/NullObject.pdf">Null Object - Something for Nothing</a></li>
 414 </ul>
 415 <p>Finally you might end up in a situation where not even the Null Object can help
 416 you. That is when you truly need <tt class="docutils literal"><span class="pre">container&lt;</span> <span class="pre">nullable&lt;T&gt;</span> <span class="pre">&gt;</span></tt>.</p>
 417 <hr><p><strong>Navigate:</strong></p>
 418 <ul class="simple">
 419 <li><a class="reference" href="ptr_container.html">home</a></li>
 420 <li><a class="reference" href="reference.html">reference</a></li>
 421 </ul>
 422 <hr><table class="docutils field-list" frame="void" rules="none">
 423 <col class="field-name" />
 424 <col class="field-body" />
 425 <tbody valign="top">
 426 <tr class="field"><th class="field-name">Copyright:</th><td class="field-body">Thorsten Ottosen 2004-2006. Use, modification and distribution is subject to the Boost Software License, Version 1.0 (see <a class="reference" href="http://www.boost.org/LICENSE_1_0.txt">LICENSE_1_0.txt</a>).</td>
 427 </tr>
 428 </tbody>
 429 </table>
 430 </div>
 431 </div>
 432 </div>
 433 </body>
 434 </html>