247 lines
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247 lines
11 KiB
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
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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@sources.redhat.com (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</TITLE>
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<LINK REL="home" HREF="http://sources.redhat.com/libstdc++/docs/23_containers/">
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<LINK REL=StyleSheet HREF="../lib3styles.css">
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<!-- $Id: howto.html,v 1.2 2000/07/11 21:45:08 pme Exp $ -->
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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>
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<LI><A HREF="#2">Variable-sized bitmasks</A>
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<LI><A HREF="#3">Containers and multithreading</A>
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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 <TT>begin()</TT> and
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<TT>end()</TT>, but not always). For builtin arrays, these are
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the address of the first element and the past-the-end element.
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<!-- a good explanation of the past-the-end rules is in order,
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probably a link somewhere
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-->
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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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<TT>beginof</TT> 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 <TT>begin()</TT>
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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 <TT>beginof</TT> 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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<PRE>
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std::transform(beginof(foo), endof(foo), beginof(foo), SomeFunction);</PRE>
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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 <TT>vector<T></TT> instead of a
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more general <TT>Container<T></TT>. This would mean that
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three functions for <TT>deque</TT> would have to be added, another
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three for <TT>list</TT>, 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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<PRE>
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inline unsigned int lengthof (T (&)[sz]) { return sz; } </PRE>
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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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<!-- 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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because <TT>n</TT> 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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<UL>
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<LI>A very large N in <TT>bitset<N></TT>.
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<LI>A container<bool>.
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<LI>Extremely weird solutions.
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</UL>
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</P>
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<P><B>A very large N in <TT>bitset<N></TT>. </B> 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 spae overhead, but it's all allocated inside the
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object.)
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</P>
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<P><B>A container<bool>. </B> The Committee 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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<TT>Container<char></TT> or <TT>Container<short int></TT>.
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Specifically, <TT>vector<bool></TT> 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 <TT>vector<bool></TT> 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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<EM>C++ Report</EM> cover it well). Future revisions of the ISO C++
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Standard will change the requirement for <TT>vector<bool></TT>
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specialization. In the meantime, <TT>deque<bool></TT> 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><B>Extremely weird solutions. </B> If you have 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 <TT>bitset</TT>
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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 Independant 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 <TT>bitset<N></TT> for the exact <TT>N</TT>
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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>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 will mention some of the problems in designing MT
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programs that use Standard containers. For information on other
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aspects of multithreading (e.g., the library as a whole), see
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the Received Wisdom on Chapter 17.
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</P>
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<P>An excellent page to read when working with templatized containers
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and threads is
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<A HREF="http://www.sgi.com/Technology/STL/thread_safety.html">SGI's
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http://www.sgi.com/Technology/STL/thread_safety.html</A>. The
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libstdc++-v3 uses the same definition of thread safety
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when discussing design. A key point that beginners may miss is the
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fourth major paragraph ("For most clients,"...), pointing
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out that locking must nearly always be done outside the container,
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by client code (that'd be you, not us *grin*).
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</P>
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<P>You didn't read it, did you? *sigh* I'm serious, go read the
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SGI page. It's really good and doesn't take long, and makes most
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of the points that would otherwise have to be made here (and does
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a better job).
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</P>
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<P>That's much better. Now, the issue of MT has been brought up on
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the libstdc++-v3 mailing list as well as the main GCC mailing list
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several times. The Chapter 17 HOWTO has some links into the mail
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archives, so you can see what's been thrown around. The usual
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container (or pseudo-container, depending on how you look at it)
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that people have in mind is <TT>string</TT>, which is one of the
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points where libstdc++ departs from the SGI STL. As of the
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2.90.8 snapshot, the libstdc++-v3 string class is safe for
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certain kinds of multithreaded access.
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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 (as SGI suggests) provide a wrapper class which obtains
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the lock, performs the container operation, then releases the lock.
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This could be templatized <EM>to a certain extent</EM>, on the
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underlying container and/or a locking mechanism. Trying to provide
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a catch-all general template solution would probably be more trouble
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than it's worth.
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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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<!-- ####################################################### -->
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<HR>
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<P CLASS="fineprint"><EM>
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Comments and suggestions are welcome, and may be sent to
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<A HREF="mailto:pme@sources.redhat.com">Phil Edwards</A> or
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<A HREF="mailto:gdr@gcc.gnu.org">Gabriel Dos Reis</A>.
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<BR> $Id: howto.html,v 1.2 2000/07/11 21:45:08 pme Exp $
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</EM></P>
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