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   4      The SGI hashing classes <code class="classname">hash_set</code> and
   5      <code class="classname">hash_set</code> have been deprecated by the
   6      unordered_set, unordered_multiset, unordered_map,
   7      unordered_multimap containers in TR1 and the upcoming C++0x, and
   8      may be removed in future releases.
   9    </p><p>The SGI headers</p><pre class="programlisting">
  10      &lt;hash_map&gt;
  11      &lt;hash_set&gt;
  12      &lt;rope&gt;
  13      &lt;slist&gt;
  14      &lt;rb_tree&gt;
  15    </pre><p>are all here;
  16       <code class="code">&lt;hash_map&gt;</code> and <code class="code">&lt;hash_set&gt;</code>
  17       are deprecated but available as backwards-compatible extensions,
  18       as discussed further below.  <code class="code">&lt;rope&gt;</code> is the
  19       SGI specialization for large strings ("rope,"
  20       "large strings," get it? Love that geeky humor.)
  21       <code class="code">&lt;slist&gt;</code> is a singly-linked list, for when the
  22       doubly-linked <code class="code">list&lt;&gt;</code> is too much space
  23       overhead, and <code class="code">&lt;rb_tree&gt;</code> exposes the red-black
  24       tree classes used in the implementation of the standard maps and
  25       sets.
  26    </p><p>Each of the associative containers map, multimap, set, and multiset
  27       have a counterpart which uses a
  28       <a class="ulink" href="http://www.sgi.com/tech/stl/HashFunction.html" target="_top">hashing
  29       function</a> to do the arranging, instead of a strict weak ordering
  30       function.  The classes take as one of their template parameters a
  31       function object that will return the hash value; by default, an
  32       instantiation of
  33       <a class="ulink" href="http://www.sgi.com/tech/stl/hash.html" target="_top">hash</a>.
  34       You should specialize this functor for your class, or define your own,
  35       before trying to use one of the hashing classes.
  36    </p><p>The hashing classes support all the usual associative container
  37       functions, as well as some extra constructors specifying the number
  38       of buckets, etc.
  39    </p><p>Why would you want to use a hashing class instead of the
  40       <span class="quote">“<span class="quote">normal</span>”</span>implementations?  Matt Austern writes:
  41    </p><div class="blockquote"><blockquote class="blockquote"><p>
  42        <span class="emphasis"><em>[W]ith a well chosen hash function, hash tables
  43        generally provide much better average-case performance than
  44        binary search trees, and much worse worst-case performance.  So
  45        if your implementation has hash_map, if you don't mind using
  46        nonstandard components, and if you aren't scared about the
  47        possibility of pathological cases, you'll probably get better
  48        performance from hash_map.
  49      </em></span>
  50      </p></blockquote></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="bk01pt03ch21s02.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ext_containers.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ext_utilities.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">HP/SGI </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 22. Utilities</td></tr></table></div></body></html>