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<title>Header boost/utility.hpp Documentation</title>
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<h1><img src="../../boost.png" alt="boost.png (6897 bytes)" align="center" WIDTH="277" HEIGHT="86">Header
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<a href="../../boost/utility.hpp">boost/utility.hpp</a></h1>
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<p>The entire contents of the header <code><a href="../../boost/utility.hpp"><boost/utility.hpp></a></code>
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are in <code>namespace boost</code>.</p>
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<h2>Contents</h2>
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<ul>
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<li>
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Class templates supporting the <a href="base_from_member.html">base-from-member
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idiom</a></li>
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<li>
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Function templates <a href="#checked_delete">checked_delete() and
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checked_array_delete()</a></li>
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<li>
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Function templates <a href="#functions_next_prior">next() and prior()</a></li>
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<li>
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Class <a href="#Class_noncopyable">noncopyable</a></li>
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<li>
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Function template <a href="#addressof">addressof()</a></li>
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<li>Class template <a href="#result_of">result_of</a></li>
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<li><a href="index.html">Other utilities not part of <code>utility.hpp</code></a></li>
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</ul>
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<h2>
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Function templates <a name="checked_delete">checked_delete</a>() and
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checked_array_delete()</h2>
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<p>See <a href="checked_delete.html">separate documentation</a>.</p>
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<h2>
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<a name="functions_next_prior">Function</a> templates next() and prior()</h2>
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<p>Certain data types, such as the C++ Standard Library's forward and bidirectional
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iterators, do not provide addition and subtraction via operator+() or
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operator-(). This means that non-modifying computation of the next or
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prior value requires a temporary, even though operator++() or operator--() is
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provided. It also means that writing code like <code>itr+1</code> inside
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a template restricts the iterator category to random access iterators.</p>
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<p>The next() and prior() functions provide a simple way around these problems:</p>
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<blockquote>
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<pre>template <class T>
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T next(T x) { return ++x; }
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template <class T, class Distance>
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T next(T x, Distance n)
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{
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std::advance(x, n);
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return x;
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}
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template <class T>
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T prior(T x) { return --x; }
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template <class T, class Distance>
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T prior(T x, Distance n)
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{
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std::advance(x, -n);
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return x;
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}</pre>
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</blockquote>
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<p>Usage is simple:</p>
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<blockquote>
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<pre>const std::list<T>::iterator p = get_some_iterator();
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const std::list<T>::iterator prev = boost::prior(p);
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const std::list<T>::iterator next = boost::next(prev, 2);</pre>
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</blockquote>
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<p>The distance from the given iterator should be supplied as an absolute value. For
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example, the iterator four iterators prior to the given iterator <code>p</code>
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may be obtained by <code>prior(p, 4)</code>.</p>
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<p>Contributed by <a href="http://www.boost.org/people/dave_abrahams.htm">Dave Abrahams</a>. Two-argument versions by Daniel Walker.</p>
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<h2><a name="Class_noncopyable">Class noncopyable</a></h2>
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<p>Class <strong>noncopyable</strong> is a base class. Derive your own class
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from <strong>noncopyable</strong> when you want to prohibit copy construction
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and copy assignment.</p>
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<p>Some objects, particularly those which hold complex resources like files or
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network connections, have no sensible copy semantics. Sometimes there are
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possible copy semantics, but these would be of very limited usefulness and be
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very difficult to implement correctly. Sometimes you're implementing a
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class that doesn't need to be copied just yet and you don't want to take the
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time to write the appropriate functions. Deriving from <b>noncopyable</b>
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will prevent the otherwise implicitly-generated functions (which don't have the
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proper semantics) from becoming a trap for other programmers.</p>
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<p>The traditional way to deal with these is to declare a private copy constructor
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and copy assignment, and then document why this is done. But deriving
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from <b>noncopyable</b> is simpler and clearer, and doesn't require additional
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documentation.</p>
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<p>The program <a href="noncopyable_test.cpp">noncopyable_test.cpp</a> can be used
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to verify class <b>noncopyable</b> works as expected. It has have been run
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successfully under GCC 2.95, Metrowerks CodeWarrior 5.0, and Microsoft Visual
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C++ 6.0 sp 3.</p>
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<p>Contributed by <a href="http://www.boost.org/people/dave_abrahams.htm">Dave Abrahams</a>.</p>
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<h3>Example</h3>
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<blockquote>
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<pre>// inside one of your own headers ...
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#include <boost/utility.hpp>
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class ResourceLadenFileSystem : boost::noncopyable {
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...</pre>
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</blockquote>
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<h3>Rationale</h3>
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<p>Class noncopyable has protected constructor and destructor members to emphasize
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that it is to be used only as a base class. Dave Abrahams notes concern
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about the effect on compiler optimization of adding (even trivial inline)
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destructor declarations. He says "Probably this concern is misplaced,
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because noncopyable will be used mostly for classes which own resources and
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thus have non-trivial destruction semantics."</p>
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<h2><a name="addressof">Function template addressof()</a></h2>
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<p>Function <strong>addressof()</strong> returns the address of an object.</p>
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<blockquote>
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<pre>template <typename T> inline T* addressof(T& v);
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template <typename T> inline const T* addressof(const T& v);
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template <typename T> inline volatile T* addressof(volatile T& v);
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template <typename T> inline const volatile T* addressof(const volatile T& v);
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</pre>
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</blockquote>
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<p>C++ allows programmers to replace the unary <strong>operator&()</strong> class
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member used to get the address of an object. Getting the real address of an
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object requires ugly casting tricks to avoid invoking the overloaded <strong>operator&()</strong>.
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Function <strong>addressof()</strong> provides a wrapper around the necessary
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code to make it easy to get an object's real address.
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</p>
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<p>The program <a href="addressof_test.cpp">addressof_test.cpp</a> can be used to
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verify that <b>addressof()</b> works as expected.</p>
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<p>Contributed by Brad King based on ideas from discussion with Doug Gregor.</p>
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<h3>Example</h3>
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<blockquote>
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<pre>#include <boost/utility.hpp>
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struct useless_type {};
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class nonaddressable {
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useless_type operator&() const;
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};
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void f() {
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nonaddressable x;
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nonaddressable* xp = boost::addressof(x);
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// nonaddressable* xpe = &x; /* error */
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}</pre>
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</blockquote>
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<h2><a name="result_of">Class template
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result_of</a></h2> <p>The class template
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<code>result_of</code> helps determine the type of a
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call expression. Given an lvalue <code>f</code> of
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type <code>F</code> and lvalues <code>t1</code>,
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<code>t2</code>, ..., <code>t<em>N</em></code> of
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types <code>T1</code>, <code>T2</code>, ...,
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<code>T<em>N</em></code>, respectively, the type
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<code>result_of<F(T1, T2, ...,
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T<em>N</em>)>::type</code> defines the result type
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of the expression <code>f(t1, t2,
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...,t<em>N</em>)</code>. The implementation permits
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the type <code>F</code> to be a function pointer,
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function reference, member function pointer, or class
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type.</p> <p>If your compiler does not support
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<code>decltype</code>, then when <code>F</code> is a
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class type with a member type <code>result_type</code>,
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<code>result_of<F(T1, T2, ...,
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T<em>N</em>)></code> is
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<code>F::result_type</code>. When <code>F</code>
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does not contain <code>result_type</code>,
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<code>result_of<F(T1, T2, ...,
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T<em>N</em>)></code> is <code>F::result<F(T1,
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T2, ..., T<em>N</em>)>::type</code> when
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<code><em>N</em> > 0</code> or <code>void</code>
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when <code><em>N</em> = 0</code>. For additional
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information about <code>result_of</code>, see the
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C++ Library Technical Report, <a
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href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1836.pdf">N1836</a>,
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or, for motivation and design rationale, the <code>result_of</code> <a
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href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1454.html">proposal</a>.</p>
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<p>Class template <code>result_of</code> resides in
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the header <code><<a
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href="../../boost/utility/result_of.hpp">boost/utility/result_of.hpp</a>></code>. By
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default, <em>N</em> may be any value between 0 and
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10. To change the upper limit, define the macro
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<code>BOOST_RESULT_OF_NUM_ARGS</code> to the maximum
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value for <em>N</em>.</p>
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<a name="BOOST_NO_RESULT_OF"></a>
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<p>This implementation of <code>result_of</code> requires class template partial specialization, the ability to parse function types properly, and support for SFINAE. If <code>result_of</code> is not supported by your compiler, including the header <code>boost/utility/result_of.hpp</code> will define the macro <code>BOOST_NO_RESULT_OF</code>. Contributed by Doug Gregor.</p>
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<h2>Class templates for the Base-from-Member Idiom</h2>
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<p>See <a href="base_from_member.html">separate documentation</a>.</p>
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<hr>
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<p>Revised <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan
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-->07 November, 2007<!--webbot bot="Timestamp" endspan i-checksum="39369"
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-->
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</p>
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<p>© Copyright Beman Dawes 1999-2003.</p>
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<p>Distributed under the Boost Software License, Version 1.0. See
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<a href="http://www.boost.org/LICENSE_1_0.txt">www.boost.org/LICENSE_1_0.txt</a></p>
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