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<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=" ISO C++ , library "/><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"><</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 <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 chaptericular order:
</p><div class="itemizedlist"><ul class="itemizedlist"><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>
<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">“<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<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
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="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<5> b ( std::string(<span class="quote">“<span class="quote">10110</span>”</span>) );
</pre><p>
instead of
</p><pre class="programlisting">
std::bitset<5> 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.
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