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   3 <html xmlns="http://www.w3.org/1999/xhtml"><head><title>Chapter 9.  Containers</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.76.1"/><meta name="keywords" content="&#10;      ISO C++&#10;    , &#10;      library&#10;    "/><meta name="keywords" content="&#10;      ISO C++&#10;    , &#10;      runtime&#10;    , &#10;      library&#10;    "/><link rel="home" href="../index.html" title="The GNU C++ Library"/><link rel="up" href="bk01pt02.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.
   4   Containers
   5
   6 </th></tr><tr><td align="left"><a accesskey="p" href="facets.html">Prev</a> </td><th width="60%" align="center">Part II.
   7     Standard Contents
   8   </th><td align="right"> <a accesskey="n" href="associative.html">Next</a></td></tr></table><hr/></div><div class="chapter" title="Chapter 9.  Containers"><div class="titlepage"><div><div><h2 class="title"><a id="std.containers"/>Chapter 9.
   9   Containers
  10   <a id="id461537" class="indexterm"/>
  11 </h2></div></div></div><div class="toc"><p><strong>Table of Contents</strong></p><dl><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="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" title="Sequences"><div class="titlepage"><div><div><h2 class="title"><a id="std.containers.sequences"/>Sequences</h2></div></div></div><div class="section" title="list"><div class="titlepage"><div><div><h3 class="title"><a id="containers.sequences.list"/>list</h3></div></div></div><div class="section" title="list::size() is O(n)"><div class="titlepage"><div><div><h4 class="title"><a id="sequences.list.size"/>list::size() is O(n)</h4></div></div></div><p>
  12      Yes it is, 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="http://www.sgi.com/tech/stl/FAQ.html">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 class="section" title="vector"><div class="titlepage"><div><div><h3 class="title"><a id="containers.sequences.vector"/>vector</h3></div></div></div><p>
  46   </p><div class="section" title="Space Overhead Management"><div class="titlepage"><div><div><h4 class="title"><a id="sequences.vector.management"/>Space Overhead Management</h4></div></div></div><p>
  47      In <a class="link" href="http://gcc.gnu.org/ml/libstdc++/2002-04/msg00105.html">this
  48      message to the list</a>, Daniel Kostecky announced work on an
  49      alternate form of <code class="code">std::vector</code> that would support
  50      hints on the number of elements to be over-allocated.  The design
  51      was also described, along with possible implementation choices.
  52    </p><p>
  53      The first two alpha releases were announced <a class="link" href="http://gcc.gnu.org/ml/libstdc++/2002-07/msg00048.html">here</a>
  54      and <a class="link" href="http://gcc.gnu.org/ml/libstdc++/2002-07/msg00111.html">here</a>.
  55    </p></div></div></div></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td align="left"><a accesskey="p" href="facets.html">Prev</a> </td><td align="center"><a accesskey="u" href="bk01pt02.html">Up</a></td><td align="right"> <a accesskey="n" href="associative.html">Next</a></td></tr><tr><td align="left" valign="top">Facets </td><td align="center"><a accesskey="h" href="../index.html">Home</a></td><td align="right" valign="top"> Associative</td></tr></table></div></body></html>