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   3      The SGI hashing classes <code class="classname">hash_set</code> and
   4      <code class="classname">hash_set</code> have been deprecated by the
   5      unordered_set, unordered_multiset, unordered_map,
   6      unordered_multimap containers in TR1 and C++11, and
   7      may be removed in future releases.
   8    </p><p>The SGI headers</p><pre class="programlisting">
   9      &lt;hash_map&gt;
  10      &lt;hash_set&gt;
  11      &lt;rope&gt;
  12      &lt;slist&gt;
  13      &lt;rb_tree&gt;
  14    </pre><p>are all here;
  15       <code class="filename">&lt;backwards/hash_map&gt;</code> and
  16       <code class="filename">&lt;backwards/hash_set&gt;</code>
  17       are deprecated but available as backwards-compatible extensions,
  18       as discussed further below.
  19       <code class="filename">&lt;ext/rope&gt;</code> is the SGI
  20       specialization for large strings ("rope," "large strings," get it? Love
  21       that geeky humor.)
  22       <code class="filename">&lt;ext/slist&gt;</code> (superseded in
  23       C++11 by <code class="filename">&lt;forward_list&gt;</code>)
  24       is a singly-linked list, for when the doubly-linked <code class="code">list&lt;&gt;</code>
  25       is too much space overhead, and
  26       <code class="filename">&lt;ext/rb_tree&gt;</code> exposes the
  27       red-black tree classes used in the implementation of the standard maps
  28       and sets.
  29    </p><p>Each of the associative containers map, multimap, set, and multiset
  30       have a counterpart which uses a
  31       <a class="link" href="http://www.sgi.com/tech/stl/HashFunction.html" target="_top">hashing
  32       function</a> to do the arranging, instead of a strict weak ordering
  33       function.  The classes take as one of their template parameters a
  34       function object that will return the hash value; by default, an
  35       instantiation of
  36       <a class="link" href="http://www.sgi.com/tech/stl/hash.html" target="_top">hash</a>.
  37       You should specialize this functor for your class, or define your own,
  38       before trying to use one of the hashing classes.
  39    </p><p>The hashing classes support all the usual associative container
  40       functions, as well as some extra constructors specifying the number
  41       of buckets, etc.
  42    </p><p>Why would you want to use a hashing class instead of the
  43       <span class="quote">“<span class="quote">normal</span>”</span>implementations?  Matt Austern writes:
  44    </p><div class="blockquote"><blockquote class="blockquote"><p>
  45        <span class="emphasis"><em>[W]ith a well chosen hash function, hash tables
  46        generally provide much better average-case performance than
  47        binary search trees, and much worse worst-case performance.  So
  48        if your implementation has hash_map, if you don't mind using
  49        nonstandard components, and if you aren't scared about the
  50        possibility of pathological cases, you'll probably get better
  51        performance from hash_map.
  52      </em></span>
  53      </p></blockquote></div><p>
  54       The deprecated hash tables are superseded by the standard unordered
  55       associative containers defined in the ISO C++ 2011 standard in the
  56       headers <code class="filename">&lt;unordered_map&gt;</code>
  57       and <code class="filename">&lt;unordered_set&gt;</code>.
  58    </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ext_containers.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">Chapter 23. HP/SGI Extensions </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 24. Utilities</td></tr></table></div></body></html>