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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="faq">FAQ</h2>
 293 <div class="contents local topic">
 294 <ul class="simple">
 295 <li><a class="reference" href="#calling-assign-is-very-costly-and-i-do-not-really-need-to-store-cloned-objects-i-merely-need-to-overwrite-the-existing-ones-what-do-i-do" id="id5" name="id5">Calling <tt class="docutils literal"><span class="pre">assign()</span></tt> is very costly and I do not really need to store cloned objects; I merely need to overwrite the existing ones; what do I do?</a></li>
 296 <li><a class="reference" href="#which-mutating-algorithms-are-safe-to-use-with-pointers" id="id6" name="id6">Which mutating algorithms are safe to use with pointers?</a></li>
 297 <li><a class="reference" href="#why-does-ptr-map-t-insert-replace-take-two-arguments-the-key-and-the-pointer-instead-of-one-std-pair-and-why-is-the-key-passed-by-non-const-reference" id="id7" name="id7">Why does <tt class="docutils literal"><span class="pre">ptr_map&lt;T&gt;::insert()/replace()</span></tt> take two arguments (the key and the pointer) instead of one <tt class="docutils literal"><span class="pre">std::pair</span></tt>? And why is the key passed by non-const reference?</a></li>
 298 <li><a class="reference" href="#when-instantiating-a-pointer-container-with-a-type-t-is-t-then-allowed-to-be-incomplete-at-that-point" id="id8" name="id8">When instantiating a pointer container with a type <tt class="docutils literal"><span class="pre">T</span></tt>, is <tt class="docutils literal"><span class="pre">T</span></tt> then allowed to be incomplete at that point?</a></li>
 299 <li><a class="reference" href="#why-do-iterator-range-inserts-give-the-strong-exception-safety-guarantee" id="id9" name="id9">Why do iterator-range inserts give the strong exception-safety guarantee?</a></li>
 300 <li><a class="reference" href="#what-is-the-polymorphic-class-problem" id="id10" name="id10">What is the polymorphic class problem?</a></li>
 301 <li><a class="reference" href="#are-the-pointer-containers-faster-and-do-they-have-a-better-memory-footprint-than-a-container-of-smart-pointers" id="id11" name="id11">Are the pointer containers faster and do they have a better memory  footprint than a container of smart pointers?</a></li>
 302 <li><a class="reference" href="#when-the-stored-pointers-cannot-be-0-how-do-i-allow-this-empty-behavior-anyway" id="id12" name="id12">When the stored pointers cannot be <tt class="docutils literal"><span class="pre">0</span></tt>, how do I allow this &quot;empty&quot; behavior anyway?</a></li>
 303 </ul>
 304 </div>
 305 <div class="section">
 306 <h1><a class="toc-backref" href="#id5" id="calling-assign-is-very-costly-and-i-do-not-really-need-to-store-cloned-objects-i-merely-need-to-overwrite-the-existing-ones-what-do-i-do" name="calling-assign-is-very-costly-and-i-do-not-really-need-to-store-cloned-objects-i-merely-need-to-overwrite-the-existing-ones-what-do-i-do">Calling <tt class="docutils literal docutils literal"><span class="pre">assign()</span></tt> is very costly and I do not really need to store cloned objects; I merely need to overwrite the existing ones; what do I do?</a></h1>
 307 <p>Call <tt class="docutils literal"><span class="pre">std::copy(</span> <span class="pre">first,</span> <span class="pre">last,</span> <span class="pre">c.begin()</span> <span class="pre">);</span></tt>.</p>
 308 </div>
 309 <div class="section">
 310 <h1><a class="toc-backref" href="#id6" id="which-mutating-algorithms-are-safe-to-use-with-pointers" name="which-mutating-algorithms-are-safe-to-use-with-pointers">Which mutating algorithms are safe to use with pointers?</a></h1>
 311 <p>Any mutating algorithm that moves elements around by swapping them.  An
 312 important example is <tt class="docutils literal"><span class="pre">std::sort()</span></tt>; examples of unsafe algorithms are
 313 <tt class="docutils literal"><span class="pre">std::unique()</span></tt> and <tt class="docutils literal"><span class="pre">std::remove()</span></tt>.</p>
 314 <!-- That is why these algorithms are
 315 provided as member functions.   -->
 316 </div>
 317 <div class="section">
 318 <h1><a class="toc-backref" href="#id7" id="why-does-ptr-map-t-insert-replace-take-two-arguments-the-key-and-the-pointer-instead-of-one-std-pair-and-why-is-the-key-passed-by-non-const-reference" name="why-does-ptr-map-t-insert-replace-take-two-arguments-the-key-and-the-pointer-instead-of-one-std-pair-and-why-is-the-key-passed-by-non-const-reference">Why does <tt class="docutils literal docutils literal"><span class="pre">ptr_map&lt;T&gt;::insert()/replace()</span></tt> take two arguments (the key and the pointer) instead of one <tt class="docutils literal docutils literal"><span class="pre">std::pair</span></tt>? And why is the key passed by non-const reference?</a></h1>
 319 <p>This is the only way the function can be implemented in an exception-safe
 320 manner; since the copy-constructor of the key might throw, and since
 321 function arguments are not guaranteed to be evaluated from left to right,
 322 we need to ensure that evaluating the first argument does not throw.
 323 Passing the key as a reference achieves just that.</p>
 324 </div>
 325 <div class="section">
 326 <h1><a class="toc-backref" href="#id8" id="when-instantiating-a-pointer-container-with-a-type-t-is-t-then-allowed-to-be-incomplete-at-that-point" name="when-instantiating-a-pointer-container-with-a-type-t-is-t-then-allowed-to-be-incomplete-at-that-point">When instantiating a pointer container with a type <tt class="docutils literal docutils literal"><span class="pre">T</span></tt>, is <tt class="docutils literal docutils literal"><span class="pre">T</span></tt> then allowed to be incomplete at that point?</a></h1>
 327 <p>No. This is a distinct property of <tt class="docutils literal"><span class="pre">shared_ptr</span></tt> which implies some overhead.</p>
 328 <p>However, one can leave <tt class="docutils literal"><span class="pre">T</span></tt> incomplete in the header file:</p>
 329 <pre class="literal-block">
 330 // foo.hpp
 331 class Foo { ... };
 332 new_clone( const Foo&amp; ) { ... }
 333 delete_clone( const Foo* )     { ... }
 334
 335 // x.hpp
 336 class Foo; // Foo is incomplete here
 337 class X { ptr_deque&lt;Foo&gt; container; ... }
 338
 339 // x.cpp
 340 #include &lt;x.hpp&gt;
 341 #include &lt;foo.hpp&gt; // now Foo is not incomplete anymore
 342 ...
 343 </pre>
 344 </div>
 345 <div class="section">
 346 <h1><a class="toc-backref" href="#id9" id="why-do-iterator-range-inserts-give-the-strong-exception-safety-guarantee" name="why-do-iterator-range-inserts-give-the-strong-exception-safety-guarantee">Why do iterator-range inserts give the strong exception-safety guarantee?</a></h1>
 347 <p>Is this not very inefficient?  It is because it is actually affordable to
 348 do so; the overhead is one heap-allocation which is relatively small
 349 compared to cloning N objects.</p>
 350 </div>
 351 <div class="section">
 352 <h1><a class="toc-backref" href="#id10" id="what-is-the-polymorphic-class-problem" name="what-is-the-polymorphic-class-problem">What is the <span class="target" id="polymorphic-class-problem">polymorphic class problem</span>?</a></h1>
 353 <p>The problem refers to the relatively troublesome way C++ supports Object
 354 Oriented programming in connection with containers of pointers to
 355 polymorphic objects.  In a language without garbage collection, you end up
 356 using either a container of smart pointers or a container that takes
 357 ownership of the pointers.  The hard part is to find a safe, fast and
 358 elegant solution.</p>
 359 </div>
 360 <div class="section">
 361 <h1><a class="toc-backref" href="#id11" id="are-the-pointer-containers-faster-and-do-they-have-a-better-memory-footprint-than-a-container-of-smart-pointers" name="are-the-pointer-containers-faster-and-do-they-have-a-better-memory-footprint-than-a-container-of-smart-pointers">Are the pointer containers faster and do they have a better memory  footprint than a container of smart pointers?</a></h1>
 362 <p>The short answer is yes: they are faster and they do use less memory; in
 363 fact, they are the only way to obtain the zero-overhead hallmark of C++.
 364 Smart pointers usually have one word or more of memory overhead per
 365 pointer because a reference count must be maintained.  And since the
 366 reference count must be maintained, there is also a runtime-overhead.  If
 367 your objects are big, then the memory overhead is often negligible, but if
 368 you have many small objects, it is not.  Further reading can be found in
 369 these references: <a class="reference" href="ptr_container.html#references">[11]</a> and <a class="reference" href="ptr_container.html#references">[12]</a>.</p>
 370 </div>
 371 <div class="section">
 372 <h1><a class="toc-backref" href="#id12" id="when-the-stored-pointers-cannot-be-0-how-do-i-allow-this-empty-behavior-anyway" name="when-the-stored-pointers-cannot-be-0-how-do-i-allow-this-empty-behavior-anyway">When the stored pointers cannot be <tt class="docutils literal docutils literal"><span class="pre">0</span></tt>, how do I allow this &quot;empty&quot; behavior anyway?</a></h1>
 373 <p>Storing a null-pointer among a list of pointers does not fit well into the Object Oriented paradigm.
 374 The most elegant design is to use the Null-Object Pattern where one basically makes a concrete
 375 class with dummy implementations of the virtual functions. See <a class="reference" href="ptr_container.html#references">[13]</a> for details.</p>
 376 <hr><table class="docutils field-list" frame="void" rules="none">
 377 <col class="field-name" />
 378 <col class="field-body" />
 379 <tbody valign="top">
 380 <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>
 381 </tr>
 382 </tbody>
 383 </table>
 384 </div>
 385 </div>
 386 </body>
 387 </html>