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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> |
