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| <html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Chapter 16. Sequences</title><meta name="generator" content="DocBook XSL Stylesheets V1.74.0" /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="containers.html" title="Part VII. Containers" /><link rel="prev" href="containers.html" title="Part VII. Containers" /><link rel="next" href="vector.html" title="vector" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 16. Sequences</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="containers.html">Prev</a> </td><th width="60%" align="center">Part VII. |
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| </th><td width="20%" align="right"> <a accesskey="n" href="vector.html">Next</a></td></tr></table><hr /></div><div class="chapter" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title"><a id="manual.containers.sequences"></a>Chapter 16. Sequences</h2></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="sect1"><a href="sequences.html#containers.sequences.list">list</a></span></dt><dd><dl><dt><span class="sect2"><a href="sequences.html#sequences.list.size">list::size() is O(n)</a></span></dt></dl></dd><dt><span class="sect1"><a href="vector.html">vector</a></span></dt><dd><dl><dt><span class="sect2"><a href="vector.html#sequences.vector.management">Space Overhead Management</a></span></dt></dl></dd></dl></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="containers.sequences.list"></a>list</h2></div></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="sequences.list.size"></a>list::size() is O(n)</h3></div></div></div><p> |
| Yes it is, and that's okay. This is a decision that we preserved |
| when we imported SGI's STL implementation. The following is |
| quoted from <a class="ulink" href="http://www.sgi.com/tech/stl/FAQ.html" target="_top">their FAQ</a>: |
| </p><div class="blockquote"><blockquote class="blockquote"><p> |
| The size() member function, for list and slist, takes time |
| proportional to the number of elements in the list. This was a |
| deliberate tradeoff. The only way to get a constant-time |
| size() for linked lists would be to maintain an extra member |
| variable containing the list's size. This would require taking |
| extra time to update that variable (it would make splice() a |
| linear time operation, for example), and it would also make the |
| list larger. Many list algorithms don't require that extra |
| word (algorithms that do require it might do better with |
| vectors than with lists), and, when it is necessary to maintain |
| an explicit size count, it's something that users can do |
| themselves. |
| </p><p> |
| This choice is permitted by the C++ standard. The standard says |
| that size() “<span class="quote">should</span>” be constant time, and |
| “<span class="quote">should</span>” does not mean the same thing as |
| “<span class="quote">shall</span>”. This is the officially recommended ISO |
| wording for saying that an implementation is supposed to do |
| something unless there is a good reason not to. |
| </p><p> |
| One implication of linear time size(): you should never write |
| </p><pre class="programlisting"> |
| if (L.size() == 0) |
| ... |
| </pre><p> |
| Instead, you should write |
| </p><pre class="programlisting"> |
| if (L.empty()) |
| ... |
| </pre></blockquote></div></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="containers.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="containers.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="vector.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Part VII. |
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