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- <meta name="AUTHOR" content="pme@gcc.gnu.org (Phil Edwards)" />
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- <meta name="DESCRIPTION" content="HOWTO for the libstdc++ chapter 23." />
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- <title>libstdc++-v3 HOWTO: Chapter 23</title>
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-
-<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>
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