1<?xml version="1.0" encoding="UTF-8" standalone="no"?> 2<!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"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Chapter 9. Containers</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.78.1" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="std_contents.html" title="Part II. Standard Contents" /><link rel="prev" href="facets.html" title="Facets" /><link rel="next" href="associative.html" title="Associative" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 9. 3 Containers 4 5</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="facets.html">Prev</a> </td><th width="60%" align="center">Part II. 6 Standard Contents 7 </th><td width="20%" align="right"> <a accesskey="n" href="associative.html">Next</a></td></tr></table><hr /></div><div class="chapter"><div class="titlepage"><div><div><h2 class="title"><a id="std.containers"></a>Chapter 9. 8 Containers 9 <a id="idm269999493408" class="indexterm"></a> 10</h2></div></div></div><div class="toc"><p><strong>Table of Contents</strong></p><dl class="toc"><dt><span class="section"><a href="containers.html#std.containers.sequences">Sequences</a></span></dt><dd><dl><dt><span class="section"><a href="containers.html#containers.sequences.list">list</a></span></dt><dd><dl><dt><span class="section"><a href="containers.html#sequences.list.size">list::size() is O(n)</a></span></dt></dl></dd><dt><span class="section"><a href="containers.html#containers.sequences.vector">vector</a></span></dt><dd><dl><dt><span class="section"><a href="containers.html#sequences.vector.management">Space Overhead Management</a></span></dt></dl></dd></dl></dd><dt><span class="section"><a href="associative.html">Associative</a></span></dt><dd><dl><dt><span class="section"><a href="associative.html#containers.associative.insert_hints">Insertion Hints</a></span></dt><dt><span class="section"><a href="associative.html#containers.associative.bitset">bitset</a></span></dt><dd><dl><dt><span class="section"><a href="associative.html#associative.bitset.size_variable">Size Variable</a></span></dt><dt><span class="section"><a href="associative.html#associative.bitset.type_string">Type String</a></span></dt></dl></dd></dl></dd><dt><span class="section"><a href="unordered_associative.html">Unordered Associative</a></span></dt><dd><dl><dt><span class="section"><a href="unordered_associative.html#containers.unordered.hash">Hash Code</a></span></dt><dd><dl><dt><span class="section"><a href="unordered_associative.html#containers.unordered.cache">Hash Code Caching Policy</a></span></dt></dl></dd></dl></dd><dt><span class="section"><a href="containers_and_c.html">Interacting with C</a></span></dt><dd><dl><dt><span class="section"><a href="containers_and_c.html#containers.c.vs_array">Containers vs. Arrays</a></span></dt></dl></dd></dl></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="std.containers.sequences"></a>Sequences</h2></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="containers.sequences.list"></a>list</h3></div></div></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="sequences.list.size"></a>list::size() is O(n)</h4></div></div></div><p> 11 Yes it is, and that's okay. This is a decision that we preserved 12 when we imported SGI's STL implementation. The following is 13 quoted from <a class="link" href="http://www.sgi.com/tech/stl/FAQ.html" target="_top">their FAQ</a>: 14 </p><div class="blockquote"><blockquote class="blockquote"><p> 15 The size() member function, for list and slist, takes time 16 proportional to the number of elements in the list. This was a 17 deliberate tradeoff. The only way to get a constant-time 18 size() for linked lists would be to maintain an extra member 19 variable containing the list's size. This would require taking 20 extra time to update that variable (it would make splice() a 21 linear time operation, for example), and it would also make the 22 list larger. Many list algorithms don't require that extra 23 word (algorithms that do require it might do better with 24 vectors than with lists), and, when it is necessary to maintain 25 an explicit size count, it's something that users can do 26 themselves. 27 </p><p> 28 This choice is permitted by the C++ standard. The standard says 29 that size() <span class="quote">“<span class="quote">should</span>”</span> be constant time, and 30 <span class="quote">“<span class="quote">should</span>”</span> does not mean the same thing as 31 <span class="quote">“<span class="quote">shall</span>”</span>. This is the officially recommended ISO 32 wording for saying that an implementation is supposed to do 33 something unless there is a good reason not to. 34 </p><p> 35 One implication of linear time size(): you should never write 36 </p><pre class="programlisting"> 37 if (L.size() == 0) 38 ... 39 </pre><p> 40 Instead, you should write 41 </p><pre class="programlisting"> 42 if (L.empty()) 43 ... 44 </pre></blockquote></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="containers.sequences.vector"></a>vector</h3></div></div></div><p> 45 </p><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="sequences.vector.management"></a>Space Overhead Management</h4></div></div></div><p> 46 In <a class="link" href="http://gcc.gnu.org/ml/libstdc++/2002-04/msg00105.html" target="_top">this 47 message to the list</a>, Daniel Kostecky announced work on an 48 alternate form of <code class="code">std::vector</code> that would support 49 hints on the number of elements to be over-allocated. The design 50 was also described, along with possible implementation choices. 51 </p><p> 52 The first two alpha releases were announced <a class="link" href="http://gcc.gnu.org/ml/libstdc++/2002-07/msg00048.html" target="_top">here</a> 53 and <a class="link" href="http://gcc.gnu.org/ml/libstdc++/2002-07/msg00111.html" target="_top">here</a>. 54 </p></div></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="facets.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="std_contents.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="associative.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Facets </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Associative</td></tr></table></div></body></html>