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| The SGI hashing classes <code class="classname">hash_set</code> and |
| <code class="classname">hash_set</code> have been deprecated by the |
| unordered_set, unordered_multiset, unordered_map, |
| unordered_multimap containers in TR1 and C++11, and |
| may be removed in future releases. |
| </p><p>The SGI headers</p><pre class="programlisting"> |
| <hash_map> |
| <hash_set> |
| <rope> |
| <slist> |
| <rb_tree> |
| </pre><p>are all here; |
| <code class="filename"><backwards/hash_map></code> and |
| <code class="filename"><backwards/hash_set></code> |
| are deprecated but available as backwards-compatible extensions, |
| as discussed further below. |
| <code class="filename"><ext/rope></code> is the SGI |
| specialization for large strings ("rope," "large strings," get it? Love |
| that geeky humor.) |
| <code class="filename"><ext/slist></code> (superseded in |
| C++11 by <code class="filename"><forward_list></code>) |
| is a singly-linked list, for when the doubly-linked <code class="code">list<></code> |
| is too much space overhead, and |
| <code class="filename"><ext/rb_tree></code> exposes the |
| red-black tree classes used in the implementation of the standard maps |
| and sets. |
| </p><p>Each of the associative containers map, multimap, set, and multiset |
| have a counterpart which uses a |
| <a class="link" href="https://web.archive.org/web/20171230172024/http://www.sgi.com/tech/stl/HashFunction.html" target="_top">hashing |
| function</a> to do the arranging, instead of a strict weak ordering |
| function. The classes take as one of their template parameters a |
| function object that will return the hash value; by default, an |
| instantiation of |
| <a class="link" href="https://web.archive.org/web/20171230172200/http://www.sgi.com/tech/stl/hash.html" target="_top">hash</a>. |
| You should specialize this functor for your class, or define your own, |
| before trying to use one of the hashing classes. |
| </p><p>The hashing classes support all the usual associative container |
| functions, as well as some extra constructors specifying the number |
| of buckets, etc. |
| </p><p>Why would you want to use a hashing class instead of the |
| <span class="quote">“<span class="quote">normal</span>”</span>implementations? Matt Austern writes: |
| </p><div class="blockquote"><blockquote class="blockquote"><p> |
| <span class="emphasis"><em>[W]ith a well chosen hash function, hash tables |
| generally provide much better average-case performance than |
| binary search trees, and much worse worst-case performance. So |
| if your implementation has hash_map, if you don't mind using |
| nonstandard components, and if you aren't scared about the |
| possibility of pathological cases, you'll probably get better |
| performance from hash_map. |
| </em></span> |
| </p></blockquote></div><p> |
| The deprecated hash tables are superseded by the standard unordered |
| associative containers defined in the ISO C++ 2011 standard in the |
| headers <code class="filename"><unordered_map></code> |
| and <code class="filename"><unordered_set></code>. |
| </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 22. 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 23. Utilities</td></tr></table></div></body></html> |
