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| Containers |
| |
| </th><td width="20%" align="right"> <a accesskey="n" href="unordered_associative.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="std.containers.associative"></a>Associative</h2></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="containers.associative.insert_hints"></a>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" target="_top">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" style="list-style-type: disc; "><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"><div class="titlepage"><div><div><h3 class="title"><a id="containers.associative.bitset"></a>bitset</h3></div></div></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="associative.bitset.size_variable"></a>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 particular order: |
| </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-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> |
| <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="ext_containers.html#manual.ext.containers.sgi" title="Backwards Compatibility">some extensions</a>. |
| </p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="associative.bitset.type_string"></a>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("10110") ); |
| </pre><p> |
| instead of |
| </p><pre class="programlisting"> |
| std::bitset<5> b ( "10110" ); // invalid |
| </pre></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="containers.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="containers.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="unordered_associative.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 9. |
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