123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213214215216217218219220221222223224225226227228229230231232233234235236237238239240241242243244245246247248249250251252253254255256257258259260261262263264265266267268269270271272273274275276277278279280281282283284285286287288289290291292293294295296297298299300301302303304305306307308309310311312313314315316317318319320321322323324325326327328329330331332333334335336337338339340341342343344345346347348349350351352353354355356357358359360361362363364365366367368369370371372373374375376377378379380381382383384385386387 |
- <?xml version="1.0" encoding="utf-8" ?>
- <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
- <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
- <head>
- <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
- <meta name="generator" content="Docutils 0.3.10: http://docutils.sourceforge.net/" />
- <title>Boost Pointer Container Library</title>
- <style type="text/css">
- /*
- :Author: David Goodger
- :Contact: goodger@users.sourceforge.net
- :Date: $Date$
- :Revision: $Revision$
- :Copyright: This stylesheet has been placed in the public domain.
- Default cascading style sheet for the HTML output of Docutils.
- See http://docutils.sf.net/docs/howto/html-stylesheets.html for how to
- customize this style sheet.
- */
- /* "! important" is used here to override other ``margin-top`` and
- ``margin-bottom`` styles that are later in the stylesheet or
- more specific. See http://www.w3.org/TR/CSS1#the-cascade */
- .first {
- margin-top: 0 ! important }
- .last, .with-subtitle {
- margin-bottom: 0 ! important }
- .hidden {
- display: none }
- a.toc-backref {
- text-decoration: none ;
- color: black }
- blockquote.epigraph {
- margin: 2em 5em ; }
- dl.docutils dd {
- margin-bottom: 0.5em }
- /* Uncomment (and remove this text!) to get bold-faced definition list terms
- dl.docutils dt {
- font-weight: bold }
- */
- div.abstract {
- margin: 2em 5em }
- div.abstract p.topic-title {
- font-weight: bold ;
- text-align: center }
- div.admonition, div.attention, div.caution, div.danger, div.error,
- div.hint, div.important, div.note, div.tip, div.warning {
- margin: 2em ;
- border: medium outset ;
- padding: 1em }
- div.admonition p.admonition-title, div.hint p.admonition-title,
- div.important p.admonition-title, div.note p.admonition-title,
- div.tip p.admonition-title {
- font-weight: bold ;
- font-family: sans-serif }
- div.attention p.admonition-title, div.caution p.admonition-title,
- div.danger p.admonition-title, div.error p.admonition-title,
- div.warning p.admonition-title {
- color: red ;
- font-weight: bold ;
- font-family: sans-serif }
- /* Uncomment (and remove this text!) to get reduced vertical space in
- compound paragraphs.
- div.compound .compound-first, div.compound .compound-middle {
- margin-bottom: 0.5em }
- div.compound .compound-last, div.compound .compound-middle {
- margin-top: 0.5em }
- */
- div.dedication {
- margin: 2em 5em ;
- text-align: center ;
- font-style: italic }
- div.dedication p.topic-title {
- font-weight: bold ;
- font-style: normal }
- div.figure {
- margin-left: 2em }
- div.footer, div.header {
- clear: both;
- font-size: smaller }
- div.line-block {
- display: block ;
- margin-top: 1em ;
- margin-bottom: 1em }
- div.line-block div.line-block {
- margin-top: 0 ;
- margin-bottom: 0 ;
- margin-left: 1.5em }
- div.sidebar {
- margin-left: 1em ;
- border: medium outset ;
- padding: 1em ;
- background-color: #ffffee ;
- width: 40% ;
- float: right ;
- clear: right }
- div.sidebar p.rubric {
- font-family: sans-serif ;
- font-size: medium }
- div.system-messages {
- margin: 5em }
- div.system-messages h1 {
- color: red }
- div.system-message {
- border: medium outset ;
- padding: 1em }
- div.system-message p.system-message-title {
- color: red ;
- font-weight: bold }
- div.topic {
- margin: 2em }
- h1.section-subtitle, h2.section-subtitle, h3.section-subtitle,
- h4.section-subtitle, h5.section-subtitle, h6.section-subtitle {
- margin-top: 0.4em }
- h1.title {
- text-align: center }
- h2.subtitle {
- text-align: center }
- hr.docutils {
- width: 75% }
- img.align-left {
- clear: left }
- img.align-right {
- clear: right }
- img.borderless {
- border: 0 }
- ol.simple, ul.simple {
- margin-bottom: 1em }
- ol.arabic {
- list-style: decimal }
- ol.loweralpha {
- list-style: lower-alpha }
- ol.upperalpha {
- list-style: upper-alpha }
- ol.lowerroman {
- list-style: lower-roman }
- ol.upperroman {
- list-style: upper-roman }
- p.attribution {
- text-align: right ;
- margin-left: 50% }
- p.caption {
- font-style: italic }
- p.credits {
- font-style: italic ;
- font-size: smaller }
- p.label {
- white-space: nowrap }
- p.rubric {
- font-weight: bold ;
- font-size: larger ;
- color: maroon ;
- text-align: center }
- p.sidebar-title {
- font-family: sans-serif ;
- font-weight: bold ;
- font-size: larger }
- p.sidebar-subtitle {
- font-family: sans-serif ;
- font-weight: bold }
- p.topic-title {
- font-weight: bold }
- pre.address {
- margin-bottom: 0 ;
- margin-top: 0 ;
- font-family: serif ;
- font-size: 100% }
- pre.line-block {
- font-family: serif ;
- font-size: 100% }
- pre.literal-block, pre.doctest-block {
- margin-left: 2em ;
- margin-right: 2em ;
- background-color: #eeeeee }
- span.classifier {
- font-family: sans-serif ;
- font-style: oblique }
- span.classifier-delimiter {
- font-family: sans-serif ;
- font-weight: bold }
- span.interpreted {
- font-family: sans-serif }
- span.option {
- white-space: nowrap }
- span.pre {
- white-space: pre }
- span.problematic {
- color: red }
- span.section-subtitle {
- /* font-size relative to parent (h1..h6 element) */
- font-size: 80% }
- table.citation {
- border-left: solid thin gray }
- table.docinfo {
- margin: 2em 4em }
- table.docutils {
- margin-top: 0.5em ;
- margin-bottom: 0.5em }
- table.footnote {
- border-left: solid thin black }
- table.docutils td, table.docutils th,
- table.docinfo td, table.docinfo th {
- padding-left: 0.5em ;
- padding-right: 0.5em ;
- vertical-align: top }
- table.docutils th.field-name, table.docinfo th.docinfo-name {
- font-weight: bold ;
- text-align: left ;
- white-space: nowrap ;
- padding-left: 0 }
- h1 tt.docutils, h2 tt.docutils, h3 tt.docutils,
- h4 tt.docutils, h5 tt.docutils, h6 tt.docutils {
- font-size: 100% }
- tt.docutils {
- background-color: #eeeeee }
- ul.auto-toc {
- list-style-type: none }
- </style>
- </head>
- <body>
- <div class="document" id="boost-pointer-container-library">
- <h1 class="title"><img alt="Boost" src="boost.png" /> Pointer Container Library</h1>
- <h2 class="subtitle" id="faq">FAQ</h2>
- <div class="contents local topic">
- <ul class="simple">
- <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>
- <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>
- <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<T>::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>
- <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>
- <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>
- <li><a class="reference" href="#what-is-the-polymorphic-class-problem" id="id10" name="id10">What is the polymorphic class problem?</a></li>
- <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>
- <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 "empty" behavior anyway?</a></li>
- </ul>
- </div>
- <div class="section">
- <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>
- <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>
- </div>
- <div class="section">
- <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>
- <p>Any mutating algorithm that moves elements around by swapping them. An
- important example is <tt class="docutils literal"><span class="pre">std::sort()</span></tt>; examples of unsafe algorithms are
- <tt class="docutils literal"><span class="pre">std::unique()</span></tt> and <tt class="docutils literal"><span class="pre">std::remove()</span></tt>.</p>
- <!-- That is why these algorithms are
- provided as member functions. -->
- </div>
- <div class="section">
- <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<T>::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>
- <p>This is the only way the function can be implemented in an exception-safe
- manner; since the copy-constructor of the key might throw, and since
- function arguments are not guaranteed to be evaluated from left to right,
- we need to ensure that evaluating the first argument does not throw.
- Passing the key as a reference achieves just that.</p>
- </div>
- <div class="section">
- <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>
- <p>No. This is a distinct property of <tt class="docutils literal"><span class="pre">shared_ptr</span></tt> which implies some overhead.</p>
- <p>However, one can leave <tt class="docutils literal"><span class="pre">T</span></tt> incomplete in the header file:</p>
- <pre class="literal-block">
- // foo.hpp
- class Foo { ... };
- new_clone( const Foo& ) { ... }
- delete_clone( const Foo* ) { ... }
- // x.hpp
- class Foo; // Foo is incomplete here
- class X { ptr_deque<Foo> container; ... }
- // x.cpp
- #include <x.hpp>
- #include <foo.hpp> // now Foo is not incomplete anymore
- ...
- </pre>
- </div>
- <div class="section">
- <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>
- <p>Is this not very inefficient? It is because it is actually affordable to
- do so; the overhead is one heap-allocation which is relatively small
- compared to cloning N objects.</p>
- </div>
- <div class="section">
- <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>
- <p>The problem refers to the relatively troublesome way C++ supports Object
- Oriented programming in connection with containers of pointers to
- polymorphic objects. In a language without garbage collection, you end up
- using either a container of smart pointers or a container that takes
- ownership of the pointers. The hard part is to find a safe, fast and
- elegant solution.</p>
- </div>
- <div class="section">
- <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>
- <p>The short answer is yes: they are faster and they do use less memory; in
- fact, they are the only way to obtain the zero-overhead hallmark of C++.
- Smart pointers usually have one word or more of memory overhead per
- pointer because a reference count must be maintained. And since the
- reference count must be maintained, there is also a runtime-overhead. If
- your objects are big, then the memory overhead is often negligible, but if
- you have many small objects, it is not. Further reading can be found in
- these references: <a class="reference" href="ptr_container.html#references">[11]</a> and <a class="reference" href="ptr_container.html#references">[12]</a>.</p>
- </div>
- <div class="section">
- <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 "empty" behavior anyway?</a></h1>
- <p>Storing a null-pointer among a list of pointers does not fit well into the Object Oriented paradigm.
- The most elegant design is to use the Null-Object Pattern where one basically makes a concrete
- class with dummy implementations of the virtual functions. See <a class="reference" href="ptr_container.html#references">[13]</a> for details.</p>
- <hr><table class="docutils field-list" frame="void" rules="none">
- <col class="field-name" />
- <col class="field-body" />
- <tbody valign="top">
- <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>
- </tr>
- </tbody>
- </table>
- </div>
- </div>
- </body>
- </html>
|