libstdc++-v3/doc/html/manual/bk01pt03ch23s02.html

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  11     "><link rel="home" href="../index.html" title="The GNU C++ Library"><link rel="up" href="ext_containers.html" title="Chapter 23. HP/SGI Extensions"><link rel="prev" href="ext_containers.html" title="Chapter 23. HP/SGI Extensions"><link rel="next" href="ext_utilities.html" title="Chapter 24. Utilities"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Deprecated</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ext_containers.html">Prev</a> </td><th width="60%" align="center">Chapter 23. HP/SGI Extensions</th><td width="20%" align="right"> <a accesskey="n" href="ext_utilities.html">Next</a></td></tr></table><hr></div><div class="section" title="Deprecated"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="manual.ext.containers.deprecated_sgi"></a>Deprecated</h2></div></div></div><p>
  12      The SGI hashing classes <code class="classname">hash_set</code> and
  13      <code class="classname">hash_set</code> have been deprecated by the
  14      unordered_set, unordered_multiset, unordered_map,
  15      unordered_multimap containers in TR1 and C++11, and
  16      may be removed in future releases.
  17    </p><p>The SGI headers</p><pre class="programlisting">
  18      &lt;hash_map&gt;
  19      &lt;hash_set&gt;
  20      &lt;rope&gt;
  21      &lt;slist&gt;
  22      &lt;rb_tree&gt;
  23    </pre><p>are all here;
  24       <code class="filename">&lt;backwards/hash_map&gt;</code> and
  25       <code class="filename">&lt;backwards/hash_set&gt;</code>
  26       are deprecated but available as backwards-compatible extensions,
  27       as discussed further below.
  28       <code class="filename">&lt;ext/rope&gt;</code> is the SGI
  29       specialization for large strings ("rope," "large strings," get it? Love
  30       that geeky humor.)
  31       <code class="filename">&lt;ext/slist&gt;</code> (superseded in
  32       C++11 by <code class="filename">&lt;forward_list&gt;</code>)
  33       is a singly-linked list, for when the doubly-linked <code class="code">list&lt;&gt;</code>
  34       is too much space overhead, and
  35       <code class="filename">&lt;ext/rb_tree&gt;</code> exposes the
  36       red-black tree classes used in the implementation of the standard maps
  37       and sets.
  38    </p><p>Each of the associative containers map, multimap, set, and multiset
  39       have a counterpart which uses a
  40       <a class="link" href="http://www.sgi.com/tech/stl/HashFunction.html" target="_top">hashing
  41       function</a> to do the arranging, instead of a strict weak ordering
  42       function.  The classes take as one of their template parameters a
  43       function object that will return the hash value; by default, an
  44       instantiation of
  45       <a class="link" href="http://www.sgi.com/tech/stl/hash.html" target="_top">hash</a>.
  46       You should specialize this functor for your class, or define your own,
  47       before trying to use one of the hashing classes.
  48    </p><p>The hashing classes support all the usual associative container
  49       functions, as well as some extra constructors specifying the number
  50       of buckets, etc.
  51    </p><p>Why would you want to use a hashing class instead of the
  52       <span class="quote">“<span class="quote">normal</span>”</span>implementations?  Matt Austern writes:
  53    </p><div class="blockquote"><blockquote class="blockquote"><p>
  54        <span class="emphasis"><em>[W]ith a well chosen hash function, hash tables
  55        generally provide much better average-case performance than
  56        binary search trees, and much worse worst-case performance.  So
  57        if your implementation has hash_map, if you don't mind using
  58        nonstandard components, and if you aren't scared about the
  59        possibility of pathological cases, you'll probably get better
  60        performance from hash_map.
  61      </em></span>
  62      </p></blockquote></div><p>
  63       The deprecated hash tables are superseded by the standard unordered
  64       associative containers defined in the ISO C++ 2011 standard in the
  65       headers <code class="filename">&lt;unordered_map&gt;</code>
  66       and <code class="filename">&lt;unordered_set&gt;</code>.
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