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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Chapter 17. Associative</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="containers.html" title="Part VII. Containers" /><link rel="prev" href="vector.html" title="vector" /><link rel="next" href="bitset.html" title="bitset" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 17. Associative</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="vector.html">Prev</a> </td><th width="60%" align="center">Part VII.
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Containers
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</th><td width="20%" align="right"> <a accesskey="n" href="bitset.html">Next</a></td></tr></table><hr /></div><div class="chapter" title="Chapter 17. Associative"><div class="titlepage"><div><div><h2 class="title"><a id="manual.containers.associative"></a>Chapter 17. Associative</h2></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="sect1"><a href="associative.html#containers.associative.insert_hints">Insertion Hints</a></span></dt><dt><span class="sect1"><a href="bitset.html">bitset</a></span></dt><dd><dl><dt><span class="sect2"><a href="bitset.html#associative.bitset.size_variable">Size Variable</a></span></dt><dt><span class="sect2"><a href="bitset.html#associative.bitset.type_string">Type String</a></span></dt></dl></dd></dl></div><div class="sect1" title="Insertion Hints"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="containers.associative.insert_hints"></a>Insertion Hints</h2></div></div></div><p>
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Section [23.1.2], Table 69, of the C++ standard lists this
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function for all of the associative containers (map, set, etc):
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</p><pre class="programlisting">
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a.insert(p,t);
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</pre><p>
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where 'p' is an iterator into the container 'a', and 't' is the
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item to insert. The standard says that <span class="quote">“<span class="quote"><code class="code">t</code> is
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inserted as close as possible to the position just prior to
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<code class="code">p</code>.</span>”</span> (Library DR #233 addresses this topic,
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referring to <a class="ulink" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1780.html" target="_top">N1780</a>.
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Since version 4.2 GCC implements the resolution to DR 233, so
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that insertions happen as close as possible to the hint. For
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earlier releases the hint was only used as described below.
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</p><p>
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Here we'll describe how the hinting works in the libstdc++
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implementation, and what you need to do in order to take
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advantage of it. (Insertions can change from logarithmic
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complexity to amortized constant time, if the hint is properly
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used.) Also, since the current implementation is based on the
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SGI STL one, these points may hold true for other library
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implementations also, since the HP/SGI code is used in a lot of
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places.
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</p><p>
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In the following text, the phrases <span class="emphasis"><em>greater
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than</em></span> and <span class="emphasis"><em>less than</em></span> refer to the
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results of the strict weak ordering imposed on the container by
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its comparison object, which defaults to (basically)
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<span class="quote">“<span class="quote"><</span>”</span>. Using those phrases is semantically sloppy,
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but I didn't want to get bogged down in syntax. I assume that if
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you are intelligent enough to use your own comparison objects,
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you are also intelligent enough to assign <span class="quote">“<span class="quote">greater</span>”</span>
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and <span class="quote">“<span class="quote">lesser</span>”</span> their new meanings in the next
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paragraph. *grin*
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</p><p>
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If the <code class="code">hint</code> parameter ('p' above) is equivalent to:
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</p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p>
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<code class="code">begin()</code>, then the item being inserted should
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have a key less than all the other keys in the container.
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The item will be inserted at the beginning of the container,
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becoming the new entry at <code class="code">begin()</code>.
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</p></li><li class="listitem"><p>
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<code class="code">end()</code>, then the item being inserted should have
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a key greater than all the other keys in the container. The
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item will be inserted at the end of the container, becoming
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the new entry before <code class="code">end()</code>.
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</p></li><li class="listitem"><p>
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neither <code class="code">begin()</code> nor <code class="code">end()</code>, then:
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Let <code class="code">h</code> be the entry in the container pointed to
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by <code class="code">hint</code>, that is, <code class="code">h = *hint</code>. Then
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the item being inserted should have a key less than that of
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<code class="code">h</code>, and greater than that of the item preceding
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<code class="code">h</code>. The new item will be inserted between
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<code class="code">h</code> and <code class="code">h</code>'s predecessor.
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</p></li></ul></div><p>
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For <code class="code">multimap</code> and <code class="code">multiset</code>, the
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restrictions are slightly looser: <span class="quote">“<span class="quote">greater than</span>”</span>
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should be replaced by <span class="quote">“<span class="quote">not less than</span>”</span>and <span class="quote">“<span class="quote">less
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than</span>”</span> should be replaced by <span class="quote">“<span class="quote">not greater
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than.</span>”</span> (Why not replace greater with
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greater-than-or-equal-to? You probably could in your head, but
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the mathematicians will tell you that it isn't the same thing.)
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</p><p>
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If the conditions are not met, then the hint is not used, and the
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insertion proceeds as if you had called <code class="code"> a.insert(t)
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</code> instead. (<span class="emphasis"><em>Note </em></span> that GCC releases
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prior to 3.0.2 had a bug in the case with <code class="code">hint ==
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begin()</code> for the <code class="code">map</code> and <code class="code">set</code>
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classes. You should not use a hint argument in those releases.)
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</p><p>
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This behavior goes well with other containers'
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<code class="code">insert()</code> functions which take an iterator: if used,
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the new item will be inserted before the iterator passed as an
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argument, same as the other containers.
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</p><p>
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<span class="emphasis"><em>Note </em></span> also that the hint in this
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implementation is a one-shot. The older insertion-with-hint
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routines check the immediately surrounding entries to ensure that
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the new item would in fact belong there. If the hint does not
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point to the correct place, then no further local searching is
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done; the search begins from scratch in logarithmic time.
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</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="vector.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="containers.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="bitset.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">vector </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> bitset</td></tr></table></div></body></html>
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