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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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