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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 V1.73.2" /><meta name="keywords" content="&#10;      ISO C++&#10;    , &#10;      library&#10;    " /><link rel="start" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="containers.html" title="Part VII. Containers" /><link rel="prev" href="bk01pt07ch16s02.html" title="vector" /><link rel="next" href="bk01pt07ch17s02.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="bk01pt07ch16s02.html">Prev</a> </td><th width="60%" align="center">Part VII. Containers</th><td width="20%" align="right"> <a accesskey="n" href="bk01pt07ch17s02.html">Next</a></td></tr></table><hr /></div><div class="chapter" lang="en" xml:lang="en"><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="bk01pt07ch17.html#containers.associative.insert_hints">Insertion Hints</a></span></dt><dt><span class="sect1"><a href="bk01pt07ch17s02.html">bitset</a></span></dt><dd><dl><dt><span class="sect2"><a href="bk01pt07ch17s02.html#associative.bitset.size_variable">Size Variable</a></span></dt><dt><span class="sect2"><a href="bk01pt07ch17s02.html#associative.bitset.type_string">Type String</a></span></dt></dl></dd></dl></div><div class="sect1" lang="en" xml:lang="en"><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>
     Section [23.1.2], Table 69, of the C++ standard lists this
     function for all of the associative containers (map, set, etc):
   </p><pre class="programlisting">
      a.insert(p,t);
   </pre><p>
     where 'p' is an iterator into the container 'a', and 't' is the
     item to insert.  The standard says that “<span class="quote"><code class="code">t</code> is
     inserted as close as possible to the position just prior to
     <code class="code">p</code>.</span>” (Library DR #233 addresses this topic,
     referring to <a class="ulink" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1780.html" target="_top">N1780</a>.
     Since version 4.2 GCC implements the resolution to DR 233, so
     that insertions happen as close as possible to the hint. For
     earlier releases the hint was only used as described below.
   </p><p>
     Here we'll describe how the hinting works in the libstdc++
     implementation, and what you need to do in order to take
     advantage of it.  (Insertions can change from logarithmic
     complexity to amortized constant time, if the hint is properly
     used.)  Also, since the current implementation is based on the
     SGI STL one, these points may hold true for other library
     implementations also, since the HP/SGI code is used in a lot of
     places.
   </p><p>
     In the following text, the phrases <span class="emphasis"><em>greater
     than</em></span> and <span class="emphasis"><em>less than</em></span> refer to the
     results of the strict weak ordering imposed on the container by
     its comparison object, which defaults to (basically)
     “<span class="quote">&lt;</span>”.  Using those phrases is semantically sloppy,
     but I didn't want to get bogged down in syntax.  I assume that if
     you are intelligent enough to use your own comparison objects,
     you are also intelligent enough to assign “<span class="quote">greater</span>”
     and “<span class="quote">lesser</span>” their new meanings in the next
     paragraph.  *grin*
   </p><p>
     If the <code class="code">hint</code> parameter ('p' above) is equivalent to:
   </p><div class="itemizedlist"><ul type="disc"><li><p>
	  <code class="code">begin()</code>, then the item being inserted should
	  have a key less than all the other keys in the container.
	  The item will be inserted at the beginning of the container,
	  becoming the new entry at <code class="code">begin()</code>.
      </p></li><li><p>
	  <code class="code">end()</code>, then the item being inserted should have
	  a key greater than all the other keys in the container.  The
	  item will be inserted at the end of the container, becoming
	  the new entry at <code class="code">end()</code>.  
      </p></li><li><p>
	  neither <code class="code">begin()</code> nor <code class="code">end()</code>, then:
	  Let <code class="code">h</code> be the entry in the container pointed to
	  by <code class="code">hint</code>, that is, <code class="code">h = *hint</code>.  Then
	  the item being inserted should have a key less than that of
	  <code class="code">h</code>, and greater than that of the item preceding
	  <code class="code">h</code>.  The new item will be inserted between
	  <code class="code">h</code> and <code class="code">h</code>'s predecessor.
	  </p></li></ul></div><p>
     For <code class="code">multimap</code> and <code class="code">multiset</code>, the
     restrictions are slightly looser: “<span class="quote">greater than</span>”
     should be replaced by “<span class="quote">not less than</span>”and “<span class="quote">less
     than</span>” should be replaced by “<span class="quote">not greater
     than.</span>” (Why not replace greater with
     greater-than-or-equal-to?  You probably could in your head, but
     the mathematicians will tell you that it isn't the same thing.)
   </p><p>
     If the conditions are not met, then the hint is not used, and the
     insertion proceeds as if you had called <code class="code"> a.insert(t)
     </code> instead.  (<span class="emphasis"><em>Note </em></span> that GCC releases
     prior to 3.0.2 had a bug in the case with <code class="code">hint ==
     begin()</code> for the <code class="code">map</code> and <code class="code">set</code>
     classes.  You should not use a hint argument in those releases.)
   </p><p>
     This behavior goes well with other containers'
     <code class="code">insert()</code> functions which take an iterator: if used,
     the new item will be inserted before the iterator passed as an
     argument, same as the other containers.
   </p><p>
     <span class="emphasis"><em>Note </em></span> also that the hint in this
     implementation is a one-shot.  The older insertion-with-hint
     routines check the immediately surrounding entries to ensure that
     the new item would in fact belong there.  If the hint does not
     point to the correct place, then no further local searching is
     done; the search begins from scratch in logarithmic time.
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