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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=" ISO C++ , library " /><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"><</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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