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   3   Containers
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   6     Standard Contents
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   8   Containers
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  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>