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diff --git a/libstdc++-v3/docs/html/23_containers/howto.html b/libstdc++-v3/docs/html/23_containers/howto.html deleted file mode 100644 index 55415affd95..00000000000 --- a/libstdc++-v3/docs/html/23_containers/howto.html +++ /dev/null @@ -1,459 +0,0 @@ -<?xml version="1.0" encoding="ISO-8859-1"?> -<!DOCTYPE html - PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" - "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> - -<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> -<head> - <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" /> - <meta name="AUTHOR" content="pme@gcc.gnu.org (Phil Edwards)" /> - <meta name="KEYWORDS" content="HOWTO, libstdc++, GCC, g++, libg++, STL" /> - <meta name="DESCRIPTION" content="HOWTO for the libstdc++ chapter 23." /> - <meta name="GENERATOR" content="vi and eight fingers" /> - <title>libstdc++-v3 HOWTO: Chapter 23: Containers</title> -<link rel="StyleSheet" href="../lib3styles.css" type="text/css" /> -<link rel="Start" href="../documentation.html" type="text/html" - title="GNU C++ Standard Library" /> -<link rel="Prev" href="../22_locale/howto.html" type="text/html" - title="Localization" /> -<link rel="Next" href="../24_iterators/howto.html" type="text/html" - title="Iterators" /> -<link rel="Copyright" href="../17_intro/license.html" type="text/html" /> -<link rel="Help" href="../faq/index.html" type="text/html" title="F.A.Q." /> -</head> -<body> - -<h1 class="centered"><a name="top">Chapter 23: Containers</a></h1> - -<p>Chapter 23 deals with container classes and what they offer. -</p> - - -<!-- ####################################################### --> -<hr /> -<h1>Contents</h1> -<ul> - <li><a href="#1">Making code unaware of the container/array difference</a></li> - <li><a href="#2">Variable-sized bitmasks</a></li> - <li><a href="#3">Containers and multithreading</a></li> - <li><a href="#4">"Hinting" during insertion</a></li> - <li><a href="#5">Bitmasks and string arguments</a></li> - <li><a href="#6"><code>std::list::size()</code> is O(n)!</a></li> - <li><a href="#7">Space overhead management for vectors</a></li> -</ul> - -<hr /> - -<!-- ####################################################### --> - -<h2><a name="1">Making code unaware of the container/array difference</a></h2> - <p>You're writing some code and can't decide whether to use builtin - arrays or some kind of container. There are compelling reasons - to use one of the container classes, but you're afraid that you'll - eventually run into difficulties, change everything back to arrays, - and then have to change all the code that uses those data types to - keep up with the change. - </p> - <p>If your code makes use of the standard algorithms, this isn't as - scary as it sounds. The algorithms don't know, nor care, about - the kind of "container" on which they work, since the - algorithms are only given endpoints to work with. For the container - classes, these are iterators (usually <code>begin()</code> and - <code>end()</code>, but not always). For builtin arrays, these are - the address of the first element and the - <a href="../24_iterators/howto.html#2">past-the-end</a> element. - </p> - <p>Some very simple wrapper functions can hide all of that from the - rest of the code. For example, a pair of functions called - <code>beginof</code> can be written, one that takes an array, another - that takes a vector. The first returns a pointer to the first - element, and the second returns the vector's <code>begin()</code> - iterator. - </p> - <p>The functions should be made template functions, and should also - be declared inline. As pointed out in the comments in the code - below, this can lead to <code>beginof</code> being optimized out of - existence, so you pay absolutely nothing in terms of increased - code size or execution time. - </p> - <p>The result is that if all your algorithm calls look like - </p> - <pre> - std::transform(beginof(foo), endof(foo), beginof(foo), SomeFunction);</pre> - <p>then the type of foo can change from an array of ints to a vector - of ints to a deque of ints and back again, without ever changing any - client code. - </p> - <p>This author has a collection of such functions, called "*of" - because they all extend the builtin "sizeof". It started - with some Usenet discussions on a transparent way to find the length - of an array. A simplified and much-reduced version for easier - reading is <a href="wrappers_h.txt">given here</a>. - </p> - <p>Astute readers will notice two things at once: first, that the - container class is still a <code>vector<T></code> instead of a - more general <code>Container<T></code>. This would mean that - three functions for <code>deque</code> would have to be added, another - three for <code>list</code>, and so on. This is due to problems with - getting template resolution correct; I find it easier just to - give the extra three lines and avoid confusion. - </p> - <p>Second, the line - </p> - <pre> - inline unsigned int lengthof (T (&)[sz]) { return sz; } </pre> - <p>looks just weird! Hint: unused parameters can be left nameless. - </p> - <p>Return <a href="#top">to top of page</a> or - <a href="../faq/index.html">to the FAQ</a>. - </p> - -<hr /> -<h2><a name="2">Variable-sized bitmasks</a></h2> - <p>No, you cannot write code of the form - </p> - <!-- Careful, the leading spaces in PRE show up directly. --> - <pre> - #include <bitset> - - void foo (size_t n) - { - std::bitset<n> bits; - .... - } </pre> - <p>because <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 - <em>is</em> 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> - <ul> - <li>A very large N in <code>bitset<N></code>.</li> - <li>A container<bool>.</li> - <li>Extremely weird solutions.</li> - </ul> - <p><strong>A very large N in - <code>bitset<N></code>. </strong> 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., - "changed"/"unchanged" flags), that's a <em>lot</em> - of state. - </p> - <p>You can then keep track of the "maximum bit used" 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><strong>A container<bool>. </strong> 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>Container<char></code> or - <code>Container<short int></code>. - Specifically, <code>vector<bool></code> is required to be - specialized for that space savings. - </p> - <p>The problem is that <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 - <u>C++ Report</u> cover it well). Future revisions of the ISO C++ - Standard will change the requirement for <code>vector<bool></code> - specialization. In the meantime, <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><strong>Extremely weird solutions. </strong> 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>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>bitset<N></code> for the exact - <code>N</code> - that you need at the time. Don't forget to delete the temporary files. - (Yes, this <em>can</em> be, and <em>has been</em>, done.) - </p> - <!-- I wonder if this next paragraph will get me in trouble... --> - <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++-v3 has - <a href="../ext/sgiexts.html#ch23">some extensions</a>. - </p> - <p>Return <a href="#top">to top of page</a> or - <a href="../faq/index.html">to the FAQ</a>. - </p> - -<hr /> -<h2><a name="3">Containers and multithreading</a></h2> - <p>This section discusses issues surrounding the design of - multithreaded applications which use Standard C++ containers. - All information in this section is current as of the gcc 3.0 - release and all later point releases. Although earlier gcc - releases had a different approach to threading configuration and - proper compilation, the basic code design rules presented here - were similar. For information on all other aspects of - multithreading as it relates to libstdc++, including details on - the proper compilation of threaded code (and compatibility between - threaded and non-threaded code), see Chapter 17. - </p> - <p>Two excellent pages to read when working with the Standard C++ - containers and threads are - <a href="http://www.sgi.com/tech/stl/thread_safety.html">SGI's - http://www.sgi.com/tech/stl/thread_safety.html</a> and - <a href="http://www.sgi.com/tech/stl/Allocators.html">SGI's - http://www.sgi.com/tech/stl/Allocators.html</a>. - </p> - <p><em>However, please ignore all discussions about the user-level - configuration of the lock implementation inside the STL - container-memory allocator on those pages. For the sake of this - discussion, libstdc++-v3 configures the SGI STL implementation, - not you. This is quite different from how gcc pre-3.0 worked. - In particular, past advice was for people using g++ to - explicitly define _PTHREADS or other macros or port-specific - compilation options on the command line to get a thread-safe - STL. This is no longer required for any port and should no - longer be done unless you really know what you are doing and - assume all responsibility.</em> - </p> - <p>Since the container implementation of libstdc++-v3 uses the SGI - code, we use the same definition of thread safety as SGI when - discussing design. A key point that beginners may miss is the - fourth major paragraph of the first page mentioned above - ("For most clients,"...), which points out that - locking must nearly always be done outside the container, by - client code (that'd be you, not us). There is a notable - exceptions to this rule. Allocators called while a container or - element is constructed uses an internal lock obtained and - released solely within libstdc++-v3 code (in fact, this is the - reason STL requires any knowledge of the thread configuration). - </p> - <p>For implementing a container which does its own locking, it is - trivial to provide a wrapper class which obtains the lock (as - SGI suggests), performs the container operation, and then - releases the lock. This could be templatized <em>to a certain - extent</em>, on the underlying container and/or a locking - mechanism. Trying to provide a catch-all general template - solution would probably be more trouble than it's worth. - </p> - <p>The STL implementation is currently configured to use the - high-speed caching memory allocator. Some people like to - test and/or normally run threaded programs with a different - default. For all details about how to globally override this - at application run-time see <a href="../ext/howto.html#3">here</a>. - </p> - <p>There is a better way (not standardized yet): It is possible to - force the malloc-based allocator on a per-case-basis for some - application code. The library team generally believes that this - is a better way to tune an application for high-speed using this - implementation of the STL. There is - <a href="../ext/howto.html#3">more information on allocators here</a>. - </p> - <p>Return <a href="#top">to top of page</a> or - <a href="../faq/index.html">to the FAQ</a>. - </p> - -<hr /> -<h2><a name="4">"Hinting" during insertion</a></h2> - <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> - 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 "iterator p is a hint - pointing to where the insert should start to search," but - specifies nothing more. (LWG Issue #233, currently in review, - addresses this topic, but I will ignore it here because it is not yet - finalized.) - </p> - <p>Here we'll describe how the hinting works in the libstdc++-v3 - 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 <em>greater than</em> and <em>less - than</em> refer to the results of the strict weak ordering imposed on - the container by its comparison object, which defaults to (basically) - "<". 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 "greater" and "lesser" - their new meanings in the next paragraph. *grin* - </p> - <p>If the <code>hint</code> parameter ('p' above) is equivalent to: - </p> - <ul> - <li><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>begin()</code>. - </li> - <li><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 - at <code>end()</code>. - </li> - <li>neither <code>begin()</code> nor <code>end()</code>, then: Let <code>h</code> - be the entry in the container pointed to by <code>hint</code>, that - is, <code>h = *hint</code>. Then the item being inserted should have - a key less than that of <code>h</code>, and greater than that of the - item preceding <code>h</code>. The new item will be inserted - between <code>h</code> and <code>h</code>'s predecessor. - </li> - </ul> - <p>For <code>multimap</code> and <code>multiset</code>, the restrictions are - slightly looser: "greater than" should be replaced by - "not less than" and "less than" should be replaced - by "not greater than." (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> a.insert(t) </code> - instead. (<strong>Note </strong> that GCC releases prior to 3.0.2 - had a bug in the case with <code>hint == begin()</code> for the - <code>map</code> and <code>set</code> classes. You should not use a hint - argument in those releases.) - </p> - <p>This behavior goes well with other container's <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. The exception - (in a sense) is with a hint of <code>end()</code>: the new item will - actually be inserted after <code>end()</code>, but it also becomes the - new <code>end()</code>. - </p> - <p><strong>Note </strong> also that the hint in this implementation is a - one-shot. The 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. (Further local searching would only increase the - time required when the hint is too far off.) - </p> - <p>Return <a href="#top">to top of page</a> or - <a href="../faq/index.html">to the FAQ</a>. - </p> - -<hr /> -<h2><a name="5">Bitmasks and string arguments</a></h2> - <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 "Links" from the - homepage, and from the C++ information "defect reflector" - 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> - std::bitset<5> b ( std::string("10110") ); - </pre> - instead of - <pre> - std::bitset<5> b ( "10110" ); // invalid - </pre> - <p>Return <a href="#top">to top of page</a> or - <a href="../faq/index.html">to the FAQ</a>. - </p> - -<hr /> -<h2><a name="6"><code>std::list::size()</code> is O(n)!</a></h2> - <p>Yes it is, and that's okay. This is a decision that we preserved when - we imported SGI's STL implementation. The following is quoted from - <a href="http://www.sgi.com/tech/stl/FAQ.html">their FAQ</a>: - </p> - <blockquote> - <p>The size() member function, for list and slist, takes time - proportional to the number of elements in the list. This was a - deliberate tradeoff. The only way to get a constant-time size() for - linked lists would be to maintain an extra member variable containing - the list's size. This would require taking extra time to update that - variable (it would make splice() a linear time operation, for example), - and it would also make the list larger. Many list algorithms don't - require that extra word (algorithms that do require it might do better - with vectors than with lists), and, when it is necessary to maintain - an explicit size count, it's something that users can do themselves. - </p> - <p>This choice is permitted by the C++ standard. The standard says that - size() "should" be constant time, and "should" - does not mean the same thing as "shall". This is the - officially recommended ISO wording for saying that an implementation - is supposed to do something unless there is a good reason not to. - </p> - <p>One implication of linear time size(): you should never write - </p> - <pre> - if (L.size() == 0) - ...</pre> - Instead, you should write - <pre> - if (L.empty()) - ...</pre> - </blockquote> - <p>Return <a href="#top">to top of page</a> or - <a href="../faq/index.html">to the FAQ</a>. - </p> - -<hr /> -<h2><a name="7">Space overhead management for vectors</a></h2> - <p>In - <a href="http://gcc.gnu.org/ml/libstdc++/2002-04/msg00105.html">this - message to the list</a>, Daniel Kostecky announced work on an - alternate form of <code>std::vector</code> that would support hints - on the number of elements to be over-allocated. The design was also - described, along with possible implementation choices. - </p> - <p>The first two alpha releases were announced - <a href="http://gcc.gnu.org/ml/libstdc++/2002-07/msg00048.html">here</a> - and - <a href="http://gcc.gnu.org/ml/libstdc++/2002-07/msg00111.html">here</a>. - The releases themselves are available at - <a href="http://www.kotelna.sk/dk/sw/caphint/"> - http://www.kotelna.sk/dk/sw/caphint/</a>. - </p> - <p>Return <a href="#top">to top of page</a> or - <a href="../faq/index.html">to the FAQ</a>. - </p> - - -<!-- ####################################################### --> - -<hr /> -<p class="fineprint"><em> -See <a href="../17_intro/license.html">license.html</a> for copying conditions. -Comments and suggestions are welcome, and may be sent to -<a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>. -</em></p> - - -</body> -</html> |