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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>bitset</title><meta name="generator" content="DocBook XSL Stylesheets V1.74.0" /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="associative.html" title="Chapter 17. Associative" /><link rel="prev" href="associative.html" title="Chapter 17. Associative" /><link rel="next" href="containers_and_c.html" title="Chapter 18. Interacting with C" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">bitset</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="associative.html">Prev</a> </td><th width="60%" align="center">Chapter 17. Associative</th><td width="20%" align="right"> <a accesskey="n" href="containers_and_c.html">Next</a></td></tr></table><hr /></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="containers.associative.bitset"></a>bitset</h2></div></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="associative.bitset.size_variable"></a>Size Variable</h3></div></div></div><p>
+ No, you cannot write code of the form
+ </p><pre class="programlisting">
+ #include <bitset>
+
+ void foo (size_t n)
+ {
+ std::bitset<n> 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 particular order:
+ </p><div class="itemizedlist"><ul type="disc"><li><p>A very large N in <code class="code">bitset<N></code>.</p></li><li><p>A container<bool>.</p></li><li><p>Extremely weird solutions.</p></li></ul></div><p>
+ <span class="emphasis"><em>A very large N in
+ <code class="code">bitset<N></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">changed</span>”/“<span class="quote">unchanged</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">maximum bit used</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<bool>. </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<char></code> or
+ <code class="code">Container<short int></code>. Specifically,
+ <code class="code">vector<bool></code> is required to be specialized for
+ that space savings.
+ </p><p>
+ The problem is that <code class="code">vector<bool></code> doesn't
+ behave like a normal vector anymore. There have been recent
+ 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<bool></code>
+ specialization. In the meantime, <code class="code">deque<bool></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<N></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="ulink" href="../ext/sgiexts.html#ch23" target="_top">some extensions</a>.
+ </p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="associative.bitset.type_string"></a>Type String</h3></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">Links</span>” from
+ the homepage, and from the C++ information “<span class="quote">defect
+ reflector</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<5> b ( std::string(“<span class="quote">10110</span>”) );
+ </pre><p>
+ instead of
+ </p><pre class="programlisting">
+ std::bitset<5> b ( “<span class="quote">10110</span>” ); // invalid
+ </pre></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="associative.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="associative.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="containers_and_c.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 17. Associative </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 18. Interacting with C</td></tr></table></div></body></html>
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