git
/
gcc
mirror of git://gcc.gnu.org/git/gcc.git
1
0
Fork 0
Code Issues Packages Projects Releases Wiki Activity
gcc/libstdc++-v3/docs/html/23_containers/howto.html

460 lines
22 KiB
HTML
Raw Blame History

<?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">&quot;Hinting&quot; 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 &quot;container&quot; 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 &quot;*of&quot;
because they all extend the builtin &quot;sizeof&quot;. 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&lt;T&gt;</code> instead of a
more general <code>Container&lt;T&gt;</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 (&amp;)[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 &lt;bitset&gt;
void foo (size_t n)
{
std::bitset&lt;n&gt; 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&lt;N&gt;</code>.</li>
<li>A container&lt;bool&gt;.</li>
<li>Extremely weird solutions.</li>
</ul>
<p><strong>A very large N in
<code>bitset&lt;N&gt;</code>.&nbsp;&nbsp;</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.,
&quot;changed&quot;/&quot;unchanged&quot; flags), that's a <em>lot</em>
of state.
</p>
<p>You can then keep track of the &quot;maximum bit used&quot; 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&lt;bool&gt;.&nbsp;&nbsp;</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&lt;char&gt;</code> or
<code>Container&lt;short int&gt;</code>.
Specifically, <code>vector&lt;bool&gt;</code> is required to be
specialized for that space savings.
</p>
<p>The problem is that <code>vector&lt;bool&gt;</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&lt;bool&gt;</code>
specialization. In the meantime, <code>deque&lt;bool&gt;</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.&nbsp;&nbsp;</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&lt;N&gt;</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
(&quot;For most clients,&quot;...), 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">&quot;Hinting&quot; 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 &quot;iterator p is a hint
pointing to where the insert should start to search,&quot; 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)
&quot;&lt;&quot;. 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 &quot;greater&quot; and &quot;lesser&quot;
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: &quot;greater than&quot; should be replaced by
&quot;not less than&quot; and &quot;less than&quot; should be replaced
by &quot;not greater than.&quot; (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 &quot;Links&quot; from the
homepage, and from the C++ information &quot;defect reflector&quot;
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&lt;5&gt; b ( std::string(&quot;10110&quot;) );
</pre>
instead of
<pre>
std::bitset&lt;5&gt; b ( &quot;10110&quot; ); // 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() &quot;should&quot; be constant time, and &quot;should&quot;
does not mean the same thing as &quot;shall&quot;. 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>
Reference in New Issue View Git Blame Copy Permalink