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| author | upstream source tree <ports@midipix.org> | 2015-03-15 20:14:05 -0400 |
|---|---|---|
| committer | upstream source tree <ports@midipix.org> | 2015-03-15 20:14:05 -0400 |
| commit | 554fd8c5195424bdbcabf5de30fdc183aba391bd (patch) | |
| tree | 976dc5ab7fddf506dadce60ae936f43f58787092 /libstdc++-v3/doc/html/manual/associative.html | |
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diff --git a/libstdc++-v3/doc/html/manual/associative.html b/libstdc++-v3/doc/html/manual/associative.html new file mode 100644 index 000000000..351ec9402 --- /dev/null +++ b/libstdc++-v3/doc/html/manual/associative.html @@ -0,0 +1,192 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> +<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. + Containers + + </td><td align="center"><a accesskey="h" href="../spine.html">Home</a></td><td align="right" valign="top"> Interacting with C</td></tr></table></div></body></html> |