bc225f98dd
2003-11-18 Jonathan Wakely <redi@gcc.gnu.org> * docs/html/configopts.html, docs/html/debug.html, docs/html/documentation.html, docs/html/explanations.html, docs/html/install.html, docs/html/17_intro/contribute.html, docs/html/17_intro/howto.html, docs/html/17_intro/license.html, docs/html/18_support/howto.html, docs/html/19_diagnostics/howto.html, docs/html/20_util/howto.html, docs/html/21_strings/howto.html, docs/html/22_locale/codecvt.html, docs/html/22_locale/ctype.html, docs/html/22_locale/howto.html, docs/html/22_locale/locale.html, docs/html/22_locale/messages.html, docs/html/23_containers/howto.html, docs/html/24_iterators/howto.html, docs/html/25_algorithms/howto.html, docs/html/26_numerics/howto.html, docs/html/27_io/howto.html, docs/html/ext/howto.html, docs/html/ext/sgiexts.html: Add <link> tags. From-SVN: r73712
460 lines
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HTML
460 lines
22 KiB
HTML
<?xml version="1.0" encoding="ISO-8859-1"?>
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<!DOCTYPE html
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PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
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<meta name="AUTHOR" content="pme@gcc.gnu.org (Phil Edwards)" />
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<meta name="KEYWORDS" content="HOWTO, libstdc++, GCC, g++, libg++, STL" />
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<meta name="DESCRIPTION" content="HOWTO for the libstdc++ chapter 23." />
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<meta name="GENERATOR" content="vi and eight fingers" />
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<title>libstdc++-v3 HOWTO: Chapter 23: Containers</title>
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<link rel="StyleSheet" href="../lib3styles.css" type="text/css" />
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<link rel="Start" href="../documentation.html" type="text/html"
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title="GNU C++ Standard Library" />
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<link rel="Prev" href="../22_locale/howto.html" type="text/html"
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title="Localization" />
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<link rel="Next" href="../24_iterators/howto.html" type="text/html"
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title="Iterators" />
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<link rel="Copyright" href="../17_intro/license.html" type="text/html" />
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<link rel="Help" href="../faq/index.html" type="text/html" title="F.A.Q." />
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</head>
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<body>
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<h1 class="centered"><a name="top">Chapter 23: Containers</a></h1>
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<p>Chapter 23 deals with container classes and what they offer.
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</p>
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<!-- ####################################################### -->
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<hr />
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<h1>Contents</h1>
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<ul>
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<li><a href="#1">Making code unaware of the container/array difference</a></li>
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<li><a href="#2">Variable-sized bitmasks</a></li>
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<li><a href="#3">Containers and multithreading</a></li>
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<li><a href="#4">"Hinting" during insertion</a></li>
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<li><a href="#5">Bitmasks and string arguments</a></li>
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<li><a href="#6"><code>std::list::size()</code> is O(n)!</a></li>
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<li><a href="#7">Space overhead management for vectors</a></li>
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</ul>
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<hr />
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<!-- ####################################################### -->
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<h2><a name="1">Making code unaware of the container/array difference</a></h2>
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<p>You're writing some code and can't decide whether to use builtin
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arrays or some kind of container. There are compelling reasons
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to use one of the container classes, but you're afraid that you'll
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eventually run into difficulties, change everything back to arrays,
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and then have to change all the code that uses those data types to
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keep up with the change.
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</p>
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<p>If your code makes use of the standard algorithms, this isn't as
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scary as it sounds. The algorithms don't know, nor care, about
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the kind of "container" on which they work, since the
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algorithms are only given endpoints to work with. For the container
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classes, these are iterators (usually <code>begin()</code> and
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<code>end()</code>, but not always). For builtin arrays, these are
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the address of the first element and the
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<a href="../24_iterators/howto.html#2">past-the-end</a> element.
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</p>
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<p>Some very simple wrapper functions can hide all of that from the
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rest of the code. For example, a pair of functions called
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<code>beginof</code> can be written, one that takes an array, another
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that takes a vector. The first returns a pointer to the first
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element, and the second returns the vector's <code>begin()</code>
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iterator.
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</p>
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<p>The functions should be made template functions, and should also
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be declared inline. As pointed out in the comments in the code
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below, this can lead to <code>beginof</code> being optimized out of
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existence, so you pay absolutely nothing in terms of increased
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code size or execution time.
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</p>
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<p>The result is that if all your algorithm calls look like
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</p>
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<pre>
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std::transform(beginof(foo), endof(foo), beginof(foo), SomeFunction);</pre>
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<p>then the type of foo can change from an array of ints to a vector
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of ints to a deque of ints and back again, without ever changing any
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client code.
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</p>
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<p>This author has a collection of such functions, called "*of"
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because they all extend the builtin "sizeof". It started
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with some Usenet discussions on a transparent way to find the length
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of an array. A simplified and much-reduced version for easier
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reading is <a href="wrappers_h.txt">given here</a>.
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</p>
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<p>Astute readers will notice two things at once: first, that the
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container class is still a <code>vector<T></code> instead of a
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more general <code>Container<T></code>. This would mean that
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three functions for <code>deque</code> would have to be added, another
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three for <code>list</code>, and so on. This is due to problems with
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getting template resolution correct; I find it easier just to
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give the extra three lines and avoid confusion.
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</p>
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<p>Second, the line
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</p>
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<pre>
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inline unsigned int lengthof (T (&)[sz]) { return sz; } </pre>
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<p>looks just weird! Hint: unused parameters can be left nameless.
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</p>
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<p>Return <a href="#top">to top of page</a> or
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<a href="../faq/index.html">to the FAQ</a>.
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</p>
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<hr />
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<h2><a name="2">Variable-sized bitmasks</a></h2>
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<p>No, you cannot write code of the form
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</p>
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<!-- Careful, the leading spaces in PRE show up directly. -->
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<pre>
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#include <bitset>
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void foo (size_t n)
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{
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std::bitset<n> bits;
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....
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} </pre>
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<p>because <code>n</code> must be known at compile time. Your compiler is
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correct; it is not a bug. That's the way templates work. (Yes, it
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<em>is</em> a feature.)
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</p>
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<p>There are a couple of ways to handle this kind of thing. Please
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consider all of them before passing judgement. They include, in
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no particular order:
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</p>
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<ul>
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<li>A very large N in <code>bitset<N></code>.</li>
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<li>A container<bool>.</li>
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<li>Extremely weird solutions.</li>
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</ul>
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<p><strong>A very large N in
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<code>bitset<N></code>. </strong> It has
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been pointed out a few times in newsgroups that N bits only takes up
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(N/8) bytes on most systems, and division by a factor of eight is pretty
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impressive when speaking of memory. Half a megabyte given over to a
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bitset (recall that there is zero space overhead for housekeeping info;
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it is known at compile time exactly how large the set is) will hold over
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four million bits. If you're using those bits as status flags (e.g.,
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"changed"/"unchanged" flags), that's a <em>lot</em>
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of state.
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</p>
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<p>You can then keep track of the "maximum bit used" during some
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testing runs on representative data, make note of how many of those bits
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really need to be there, and then reduce N to a smaller number. Leave
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some extra space, of course. (If you plan to write code like the
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incorrect example above, where the bitset is a local variable, then you
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may have to talk your compiler into allowing that much stack space;
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there may be zero space overhead, but it's all allocated inside the
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object.)
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</p>
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<p><strong>A container<bool>. </strong> The Committee
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made provision
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for the space savings possible with that (N/8) usage previously mentioned,
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so that you don't have to do wasteful things like
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<code>Container<char></code> or
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<code>Container<short int></code>.
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Specifically, <code>vector<bool></code> is required to be
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specialized for that space savings.
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</p>
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<p>The problem is that <code>vector<bool></code> doesn't behave like a
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normal vector anymore. There have been recent journal articles which
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discuss the problems (the ones by Herb Sutter in the May and
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July/August 1999 issues of
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<u>C++ Report</u> cover it well). Future revisions of the ISO C++
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Standard will change the requirement for <code>vector<bool></code>
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specialization. In the meantime, <code>deque<bool></code> is
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recommended (although its behavior is sane, you probably will not get
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the space savings, but the allocation scheme is different than that
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of vector).
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</p>
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<p><strong>Extremely weird solutions. </strong> If you have
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access to
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the compiler and linker at runtime, you can do something insane, like
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figuring out just how many bits you need, then writing a temporary
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source code file. That file contains an instantiation of
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<code>bitset</code>
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for the required number of bits, inside some wrapper functions with
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unchanging signatures. Have your program then call the
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compiler on that file using Position Independent Code, then open the
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newly-created object file and load those wrapper functions. You'll have
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an instantiation of <code>bitset<N></code> for the exact
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<code>N</code>
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that you need at the time. Don't forget to delete the temporary files.
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(Yes, this <em>can</em> be, and <em>has been</em>, done.)
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</p>
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<!-- I wonder if this next paragraph will get me in trouble... -->
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<p>This would be the approach of either a visionary genius or a raving
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lunatic, depending on your programming and management style. Probably
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the latter.
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</p>
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<p>Which of the above techniques you use, if any, are up to you and your
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intended application. Some time/space profiling is indicated if it
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really matters (don't just guess). And, if you manage to do anything
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along the lines of the third category, the author would love to hear
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from you...
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</p>
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<p>Also note that the implementation of bitset used in libstdc++-v3 has
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<a href="../ext/sgiexts.html#ch23">some extensions</a>.
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</p>
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<p>Return <a href="#top">to top of page</a> or
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<a href="../faq/index.html">to the FAQ</a>.
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</p>
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<hr />
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<h2><a name="3">Containers and multithreading</a></h2>
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<p>This section discusses issues surrounding the design of
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multithreaded applications which use Standard C++ containers.
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All information in this section is current as of the gcc 3.0
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release and all later point releases. Although earlier gcc
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releases had a different approach to threading configuration and
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proper compilation, the basic code design rules presented here
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were similar. For information on all other aspects of
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multithreading as it relates to libstdc++, including details on
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the proper compilation of threaded code (and compatibility between
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threaded and non-threaded code), see Chapter 17.
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</p>
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<p>Two excellent pages to read when working with the Standard C++
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containers and threads are
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<a href="http://www.sgi.com/tech/stl/thread_safety.html">SGI's
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http://www.sgi.com/tech/stl/thread_safety.html</a> and
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<a href="http://www.sgi.com/tech/stl/Allocators.html">SGI's
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http://www.sgi.com/tech/stl/Allocators.html</a>.
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</p>
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<p><em>However, please ignore all discussions about the user-level
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configuration of the lock implementation inside the STL
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container-memory allocator on those pages. For the sake of this
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discussion, libstdc++-v3 configures the SGI STL implementation,
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not you. This is quite different from how gcc pre-3.0 worked.
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In particular, past advice was for people using g++ to
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explicitly define _PTHREADS or other macros or port-specific
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compilation options on the command line to get a thread-safe
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STL. This is no longer required for any port and should no
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longer be done unless you really know what you are doing and
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assume all responsibility.</em>
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</p>
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<p>Since the container implementation of libstdc++-v3 uses the SGI
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code, we use the same definition of thread safety as SGI when
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discussing design. A key point that beginners may miss is the
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fourth major paragraph of the first page mentioned above
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("For most clients,"...), which points out that
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locking must nearly always be done outside the container, by
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client code (that'd be you, not us). There is a notable
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exceptions to this rule. Allocators called while a container or
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element is constructed uses an internal lock obtained and
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released solely within libstdc++-v3 code (in fact, this is the
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reason STL requires any knowledge of the thread configuration).
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</p>
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<p>For implementing a container which does its own locking, it is
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trivial to provide a wrapper class which obtains the lock (as
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SGI suggests), performs the container operation, and then
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releases the lock. This could be templatized <em>to a certain
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extent</em>, on the underlying container and/or a locking
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mechanism. Trying to provide a catch-all general template
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solution would probably be more trouble than it's worth.
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</p>
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<p>The STL implementation is currently configured to use the
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high-speed caching memory allocator. Some people like to
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test and/or normally run threaded programs with a different
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default. For all details about how to globally override this
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at application run-time see <a href="../ext/howto.html#3">here</a>.
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</p>
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<p>There is a better way (not standardized yet): It is possible to
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force the malloc-based allocator on a per-case-basis for some
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application code. The library team generally believes that this
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is a better way to tune an application for high-speed using this
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implementation of the STL. There is
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<a href="../ext/howto.html#3">more information on allocators here</a>.
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</p>
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<p>Return <a href="#top">to top of page</a> or
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<a href="../faq/index.html">to the FAQ</a>.
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</p>
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<hr />
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<h2><a name="4">"Hinting" during insertion</a></h2>
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<p>Section [23.1.2], Table 69, of the C++ standard lists this function
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for all of the associative containers (map, set, etc):
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</p>
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<pre>
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a.insert(p,t);</pre>
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<p>where 'p' is an iterator into the container 'a', and 't' is the item
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to insert. The standard says that "iterator p is a hint
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pointing to where the insert should start to search," but
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specifies nothing more. (LWG Issue #233, currently in review,
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addresses this topic, but I will ignore it here because it is not yet
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finalized.)
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</p>
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<p>Here we'll describe how the hinting works in the libstdc++-v3
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implementation, and what you need to do in order to take advantage of
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it. (Insertions can change from logarithmic complexity to amortized
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constant time, if the hint is properly used.) Also, since the current
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implementation is based on the SGI STL one, these points may hold true
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for other library implementations also, since the HP/SGI code is used
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in a lot of places.
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</p>
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<p>In the following text, the phrases <em>greater than</em> and <em>less
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than</em> refer to the results of the strict weak ordering imposed on
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the container by its comparison object, which defaults to (basically)
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"<". Using those phrases is semantically sloppy, but I
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didn't want to get bogged down in syntax. I assume that if you are
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intelligent enough to use your own comparison objects, you are also
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intelligent enough to assign "greater" and "lesser"
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their new meanings in the next paragraph. *grin*
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</p>
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<p>If the <code>hint</code> parameter ('p' above) is equivalent to:
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</p>
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<ul>
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<li><code>begin()</code>, then the item being inserted should have a key
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less than all the other keys in the container. The item will
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be inserted at the beginning of the container, becoming the new
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entry at <code>begin()</code>.
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</li>
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<li><code>end()</code>, then the item being inserted should have a key
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greater than all the other keys in the container. The item will
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be inserted at the end of the container, becoming the new entry
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at <code>end()</code>.
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</li>
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<li>neither <code>begin()</code> nor <code>end()</code>, then: Let <code>h</code>
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be the entry in the container pointed to by <code>hint</code>, that
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is, <code>h = *hint</code>. Then the item being inserted should have
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a key less than that of <code>h</code>, and greater than that of the
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item preceding <code>h</code>. The new item will be inserted
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between <code>h</code> and <code>h</code>'s predecessor.
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</li>
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</ul>
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<p>For <code>multimap</code> and <code>multiset</code>, the restrictions are
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slightly looser: "greater than" should be replaced by
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"not less than" and "less than" should be replaced
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by "not greater than." (Why not replace greater with
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greater-than-or-equal-to? You probably could in your head, but the
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mathematicians will tell you that it isn't the same thing.)
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</p>
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<p>If the conditions are not met, then the hint is not used, and the
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insertion proceeds as if you had called <code> a.insert(t) </code>
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instead. (<strong>Note </strong> that GCC releases prior to 3.0.2
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had a bug in the case with <code>hint == begin()</code> for the
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<code>map</code> and <code>set</code> classes. You should not use a hint
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argument in those releases.)
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</p>
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<p>This behavior goes well with other container's <code>insert()</code>
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functions which take an iterator: if used, the new item will be
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inserted before the iterator passed as an argument, same as the other
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containers. The exception
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(in a sense) is with a hint of <code>end()</code>: the new item will
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actually be inserted after <code>end()</code>, but it also becomes the
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new <code>end()</code>.
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</p>
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<p><strong>Note </strong> also that the hint in this implementation is a
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one-shot. The insertion-with-hint routines check the immediately
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surrounding entries to ensure that the new item would in fact belong
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there. If the hint does not point to the correct place, then no
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further local searching is done; the search begins from scratch in
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logarithmic time. (Further local searching would only increase the
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time required when the hint is too far off.)
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</p>
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<p>Return <a href="#top">to top of page</a> or
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<a href="../faq/index.html">to the FAQ</a>.
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</p>
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<hr />
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<h2><a name="5">Bitmasks and string arguments</a></h2>
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<p>Bitmasks do not take char* nor const char* arguments in their
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constructors. This is something of an accident, but you can read
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about the problem: follow the library's "Links" from the
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homepage, and from the C++ information "defect reflector"
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link, select the library issues list. Issue number 116 describes the
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problem.
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</p>
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<p>For now you can simply make a temporary string object using the
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constructor expression:
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</p>
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<pre>
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std::bitset<5> b ( std::string("10110") );
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</pre>
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instead of
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<pre>
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std::bitset<5> b ( "10110" ); // invalid
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</pre>
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<p>Return <a href="#top">to top of page</a> or
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<a href="../faq/index.html">to the FAQ</a>.
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</p>
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<hr />
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<h2><a name="6"><code>std::list::size()</code> is O(n)!</a></h2>
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<p>Yes it is, and that's okay. This is a decision that we preserved when
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we imported SGI's STL implementation. The following is quoted from
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<a href="http://www.sgi.com/tech/stl/FAQ.html">their FAQ</a>:
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</p>
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<blockquote>
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|
<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>
|
|
|
|
|
|
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|
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|
|
<p class="fineprint"><em>
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|
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>.
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</em></p>
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