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jeremybenn |
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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 V1.75.2" /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="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" title="bitset"><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" title="Size Variable"><div class="titlepage"><div><div><h3 class="title"><a id="associative.bitset.size_variable"></a>Size Variable</h3></div></div></div><p>
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No, you cannot write code of the form
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</p><pre class="programlisting">
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#include <bitset>
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void foo (size_t n)
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{
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std::bitset<n> bits;
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....
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}
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</pre><p>
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because <code class="code">n</code> must be known at compile time. Your
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compiler is correct; it is not a bug. That's the way templates
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work. (Yes, it <span class="emphasis"><em>is</em></span> a feature.)
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</p><p>
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There are a couple of ways to handle this kind of thing. Please
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consider all of them before passing judgement. They include, in
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no particular order:
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</p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p>A very large N in <code class="code">bitset<N></code>.</p></li><li class="listitem"><p>A container<bool>.</p></li><li class="listitem"><p>Extremely weird solutions.</p></li></ul></div><p>
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<span class="emphasis"><em>A very large N in
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<code class="code">bitset<N></code>. </em></span> It has been
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pointed out a few times in newsgroups that N bits only takes up
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(N/8) bytes on most systems, and division by a factor of eight is
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pretty impressive when speaking of memory. Half a megabyte given
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over to a bitset (recall that there is zero space overhead for
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housekeeping info; it is known at compile time exactly how large
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the set is) will hold over four million bits. If you're using
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those bits as status flags (e.g.,
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<span class="quote">“<span class="quote">changed</span>”</span>/<span class="quote">“<span class="quote">unchanged</span>”</span> flags), that's a
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<span class="emphasis"><em>lot</em></span> of state.
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</p><p>
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You can then keep track of the <span class="quote">“<span class="quote">maximum bit used</span>”</span>
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during some testing runs on representative data, make note of how
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many of those bits really need to be there, and then reduce N to
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a smaller number. Leave some extra space, of course. (If you
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plan to write code like the incorrect example above, where the
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bitset is a local variable, then you may have to talk your
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compiler into allowing that much stack space; there may be zero
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space overhead, but it's all allocated inside the object.)
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</p><p>
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<span class="emphasis"><em>A container<bool>. </em></span> The
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Committee made provision for the space savings possible with that
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(N/8) usage previously mentioned, so that you don't have to do
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wasteful things like <code class="code">Container<char></code> or
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<code class="code">Container<short int></code>. Specifically,
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<code class="code">vector<bool></code> is required to be specialized for
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that space savings.
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</p><p>
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The problem is that <code class="code">vector<bool></code> doesn't
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behave like a normal vector anymore. There have been
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journal articles which discuss the problems (the ones by Herb
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Sutter in the May and July/August 1999 issues of C++ Report cover
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it well). Future revisions of the ISO C++ Standard will change
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the requirement for <code class="code">vector<bool></code>
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specialization. In the meantime, <code class="code">deque<bool></code>
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is recommended (although its behavior is sane, you probably will
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not get the space savings, but the allocation scheme is different
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than that of vector).
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</p><p>
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<span class="emphasis"><em>Extremely weird solutions. </em></span> If
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you have access to the compiler and linker at runtime, you can do
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something insane, like figuring out just how many bits you need,
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then writing a temporary source code file. That file contains an
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instantiation of <code class="code">bitset</code> for the required number of
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bits, inside some wrapper functions with unchanging signatures.
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Have your program then call the compiler on that file using
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Position Independent Code, then open the newly-created object
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file and load those wrapper functions. You'll have an
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instantiation of <code class="code">bitset<N></code> for the exact
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<code class="code">N</code> that you need at the time. Don't forget to delete
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the temporary files. (Yes, this <span class="emphasis"><em>can</em></span> be, and
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<span class="emphasis"><em>has been</em></span>, done.)
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</p><p>
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This would be the approach of either a visionary genius or a
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raving lunatic, depending on your programming and management
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style. Probably the latter.
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</p><p>
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Which of the above techniques you use, if any, are up to you and
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your intended application. Some time/space profiling is
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indicated if it really matters (don't just guess). And, if you
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manage to do anything along the lines of the third category, the
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author would love to hear from you...
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</p><p>
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Also note that the implementation of bitset used in libstdc++ has
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<a class="link" href="bk01pt12ch34s02.html" title="HP/SGI">some extensions</a>.
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</p></div><div class="sect2" title="Type String"><div class="titlepage"><div><div><h3 class="title"><a id="associative.bitset.type_string"></a>Type String</h3></div></div></div><p>
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</p><p>
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Bitmasks do not take char* nor const char* arguments in their
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constructors. This is something of an accident, but you can read
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about the problem: follow the library's <span class="quote">“<span class="quote">Links</span>”</span> from
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the homepage, and from the C++ information <span class="quote">“<span class="quote">defect
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reflector</span>”</span> link, select the library issues list. Issue
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number 116 describes the problem.
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</p><p>
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For now you can simply make a temporary string object using the
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constructor expression:
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</p><pre class="programlisting">
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std::bitset<5> b ( std::string(<span class="quote">“<span class="quote">10110</span>”</span>) );
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</pre><p>
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instead of
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</p><pre class="programlisting">
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std::bitset<5> b ( <span class="quote">“<span class="quote">10110</span>”</span> ); // invalid
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</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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