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+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
+<html xmlns="http://www.w3.org/1999/xhtml"><head><title>Associative</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.76.1"/><meta name="keywords" content="&#10; ISO C++&#10; , &#10; library&#10; "/><link rel="home" href="../spine.html" title="The GNU C++ Library"/><link rel="up" href="containers.html" title="Chapter 9.  Containers"/><link rel="prev" href="containers.html" title="Chapter 9.  Containers"/><link rel="next" href="containers_and_c.html" title="Interacting with C"/></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Associative</th></tr><tr><td align="left"><a accesskey="p" href="containers.html">Prev</a> </td><th width="60%" align="center">Chapter 9. 
+ Containers
+
+</th><td align="right"> <a accesskey="n" href="containers_and_c.html">Next</a></td></tr></table><hr/></div><div class="section" title="Associative"><div class="titlepage"><div><div><h2 class="title"><a id="std.containers.associative"/>Associative</h2></div></div></div><div class="section" title="Insertion Hints"><div class="titlepage"><div><div><h3 class="title"><a id="containers.associative.insert_hints"/>Insertion Hints</h3></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">“<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>”</span> (Library DR #233 addresses this topic,
+ referring to <a class="link" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1780.html">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 class="quote">&lt;</span>”</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">“<span class="quote">greater</span>”</span>
+ and <span class="quote">“<span class="quote">lesser</span>”</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 class="itemizedlist"><li class="listitem"><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 class="listitem"><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 before <code class="code">end()</code>.
+ </p></li><li class="listitem"><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">“<span class="quote">greater than</span>”</span>
+ should be replaced by <span class="quote">“<span class="quote">not less than</span>”</span>and <span class="quote">“<span class="quote">less
+ than</span>”</span> should be replaced by <span class="quote">“<span class="quote">not greater
+ than.</span>”</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.
+ </p></div><div class="section" title="bitset"><div class="titlepage"><div><div><h3 class="title"><a id="containers.associative.bitset"/>bitset</h3></div></div></div><div class="section" title="Size Variable"><div class="titlepage"><div><div><h4 class="title"><a id="associative.bitset.size_variable"/>Size Variable</h4></div></div></div><p>
+ No, you cannot write code of the form
+ </p><pre class="programlisting">
+ #include &lt;bitset&gt;
+
+ void foo (size_t n)
+ {
+ std::bitset&lt;n&gt; bits;
+ ....
+ }
+ </pre><p>
+ because <code class="code">n</code> must be known at compile time. Your
+ compiler is correct; it is not a bug. That's the way templates
+ work. (Yes, it <span class="emphasis"><em>is</em></span> a feature.)
+ </p><p>
+ There are a couple of ways to handle this kind of thing. Please
+ consider all of them before passing judgement. They include, in
+ no chaptericular order:
+ </p><div class="itemizedlist"><ul class="itemizedlist"><li class="listitem"><p>A very large N in <code class="code">bitset&lt;N&gt;</code>.</p></li><li class="listitem"><p>A container&lt;bool&gt;.</p></li><li class="listitem"><p>Extremely weird solutions.</p></li></ul></div><p>
+ <span class="emphasis"><em>A very large N in
+ <code class="code">bitset&lt;N&gt;</code>.  </em></span> It has been
+ pointed out a few times in newsgroups that N bits only takes up
+ (N/8) bytes on most systems, and division by a factor of eight is
+ pretty impressive when speaking of memory. Half a megabyte given
+ over to a bitset (recall that there is zero space overhead for
+ housekeeping info; it is known at compile time exactly how large
+ the set is) will hold over four million bits. If you're using
+ those bits as status flags (e.g.,
+ <span class="quote">“<span class="quote">changed</span>”</span>/<span class="quote">“<span class="quote">unchanged</span>”</span> flags), that's a
+ <span class="emphasis"><em>lot</em></span> of state.
+ </p><p>
+ You can then keep track of the <span class="quote">“<span class="quote">maximum bit used</span>”</span>
+ during some testing runs on representative data, make note of how
+ many of those bits really need to be there, and then reduce N to
+ a smaller number. Leave some extra space, of course. (If you
+ plan to write code like the incorrect example above, where the
+ bitset is a local variable, then you may have to talk your
+ compiler into allowing that much stack space; there may be zero
+ space overhead, but it's all allocated inside the object.)
+ </p><p>
+ <span class="emphasis"><em>A container&lt;bool&gt;.  </em></span> The
+ Committee made provision for the space savings possible with that
+ (N/8) usage previously mentioned, so that you don't have to do
+ wasteful things like <code class="code">Container&lt;char&gt;</code> or
+ <code class="code">Container&lt;short int&gt;</code>. Specifically,
+ <code class="code">vector&lt;bool&gt;</code> is required to be specialized for
+ that space savings.
+ </p><p>
+ The problem is that <code class="code">vector&lt;bool&gt;</code> doesn't
+ behave like a normal vector anymore. There have been
+ journal articles which discuss the problems (the ones by Herb
+ Sutter in the May and July/August 1999 issues of C++ Report cover
+ it well). Future revisions of the ISO C++ Standard will change
+ the requirement for <code class="code">vector&lt;bool&gt;</code>
+ specialization. In the meantime, <code class="code">deque&lt;bool&gt;</code>
+ is recommended (although its behavior is sane, you probably will
+ not get the space savings, but the allocation scheme is different
+ than that of vector).
+ </p><p>
+ <span class="emphasis"><em>Extremely weird solutions.  </em></span> If
+ you have access to the compiler and linker at runtime, you can do
+ something insane, like figuring out just how many bits you need,
+ then writing a temporary source code file. That file contains an
+ instantiation of <code class="code">bitset</code> for the required number of
+ bits, inside some wrapper functions with unchanging signatures.
+ Have your program then call the compiler on that file using
+ Position Independent Code, then open the newly-created object
+ file and load those wrapper functions. You'll have an
+ instantiation of <code class="code">bitset&lt;N&gt;</code> for the exact
+ <code class="code">N</code> that you need at the time. Don't forget to delete
+ the temporary files. (Yes, this <span class="emphasis"><em>can</em></span> be, and
+ <span class="emphasis"><em>has been</em></span>, done.)
+ </p><p>
+ This would be the approach of either a visionary genius or a
+ raving lunatic, depending on your programming and management
+ style. Probably the latter.
+ </p><p>
+ Which of the above techniques you use, if any, are up to you and
+ your intended application. Some time/space profiling is
+ indicated if it really matters (don't just guess). And, if you
+ manage to do anything along the lines of the third category, the
+ author would love to hear from you...
+ </p><p>
+ Also note that the implementation of bitset used in libstdc++ has
+ <a class="link" href="bk01pt03ch21s02.html" title="HP/SGI">some extensions</a>.
+ </p></div><div class="section" title="Type String"><div class="titlepage"><div><div><h4 class="title"><a id="associative.bitset.type_string"/>Type String</h4></div></div></div><p>
+ </p><p>
+ Bitmasks do not take char* nor const char* arguments in their
+ constructors. This is something of an accident, but you can read
+ about the problem: follow the library's <span class="quote">“<span class="quote">Links</span>”</span> from
+ the homepage, and from the C++ information <span class="quote">“<span class="quote">defect
+ reflector</span>”</span> link, select the library issues list. Issue
+ number 116 describes the problem.
+ </p><p>
+ For now you can simply make a temporary string object using the
+ constructor expression:
+ </p><pre class="programlisting">
+ std::bitset&lt;5&gt; b ( std::string(<span class="quote">“<span class="quote">10110</span>”</span>) );
+ </pre><p>
+ instead of
+ </p><pre class="programlisting">
+ std::bitset&lt;5&gt; b ( <span class="quote">“<span class="quote">10110</span>”</span> ); // invalid
+ </pre></div></div></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td align="left"><a accesskey="p" href="containers.html">Prev</a> </td><td align="center"><a accesskey="u" href="containers.html">Up</a></td><td align="right"> <a accesskey="n" href="containers_and_c.html">Next</a></td></tr><tr><td align="left" valign="top">Chapter 9. 
+ Containers
+
+ </td><td align="center"><a accesskey="h" href="../spine.html">Home</a></td><td align="right" valign="top"> Interacting with C</td></tr></table></div></body></html>