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>Chapter9. Containers</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><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="PartII. 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">Chapter9. 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">PartII. 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>Chapter9. 8 Containers 9 <a id="id-1.3.4.7.1.1.1" 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></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.insert_hints">Insertion Hints</a></span></dt><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, at least using the <a class="link" href="using_dual_abi.html" title="Dual ABI">old 12 ABI</a>, and that's okay. This is a decision that we preserved 13 when we imported SGI's STL implementation. The following is 14 quoted from <a class="link" href="https://web.archive.org/web/20161222192301/http://www.sgi.com/tech/stl/FAQ.html" target="_top">their FAQ</a>: 15 </p><div class="blockquote"><blockquote class="blockquote"><p> 16 The size() member function, for list and slist, takes time 17 proportional to the number of elements in the list. This was a 18 deliberate tradeoff. The only way to get a constant-time 19 size() for linked lists would be to maintain an extra member 20 variable containing the list's size. This would require taking 21 extra time to update that variable (it would make splice() a 22 linear time operation, for example), and it would also make the 23 list larger. Many list algorithms don't require that extra 24 word (algorithms that do require it might do better with 25 vectors than with lists), and, when it is necessary to maintain 26 an explicit size count, it's something that users can do 27 themselves. 28 </p><p> 29 This choice is permitted by the C++ standard. The standard says 30 that size() <span class="quote"><span class="quote">should</span></span> be constant time, and 31 <span class="quote"><span class="quote">should</span></span> does not mean the same thing as 32 <span class="quote"><span class="quote">shall</span></span>. This is the officially recommended ISO 33 wording for saying that an implementation is supposed to do 34 something unless there is a good reason not to. 35 </p><p> 36 One implication of linear time size(): you should never write 37 </p><pre class="programlisting"> 38 if (L.size() == 0) 39 ... 40 </pre><p> 41 Instead, you should write 42 </p><pre class="programlisting"> 43 if (L.empty()) 44 ... 45 </pre></blockquote></div></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>
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