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264 lines
8.5 KiB
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<title>Boost Function Object Adapter Library</title>
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<body bgcolor="#FFFFFF" text="#000000">
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<table border="1" bgcolor="#007F7F" cellpadding="2" summary="">
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<td bgcolor="#FFFFFF"><img src="../../boost.png" alt=
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"boost.png (6897 bytes)" width="277" height="86"></td>
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<td><a href="../../index.htm"><font face="Arial" color=
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"#FFFFFF"><big>Home</big></font></a></td>
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<td><a href="../libraries.htm"><font face="Arial" color=
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"#FFFFFF"><big>Libraries</big></font></a></td>
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<td><a href="http://www.boost.org/people/people.htm"><font face="Arial" color=
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"#FFFFFF"><big>People</big></font></a></td>
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<td><a href="http://www.boost.org/more/faq.htm"><font face="Arial" color=
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<td><a href="../../more/index.htm"><font face="Arial" color=
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"#FFFFFF"><big>More</big></font></a></td>
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</table>
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<h1>Improved Function Object Adapters</h1>
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<p>The header <a href="../../boost/functional.hpp">functional.hpp</a>
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provides enhancements to the function object adapters specified in the C++
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Standard Library (sections 20.3.5, through to 20.3.8). The enhancements are
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principally possible due to two changes:</p>
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<ol>
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<li>We use the Boost <tt><a href=
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"../utility/call_traits.htm">call_traits</a></tt> templates to avoid the
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problem of <a href="binders.html#refref">references to references</a>,
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and to improve the efficiency of <a href="mem_fun.html#args">parameter
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passing</a>.</li>
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<li>We use two <a href="function_traits.html">function object traits</a>
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class templates to avoid the need for <tt><a href=
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"ptr_fun.html">ptr_fun</a></tt> with the adapters in this library.</li>
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</ol>
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<h3>Contents</h3>
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<p>The header contains the following function and class templates:</p>
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<table border="1" cellpadding="5" summary="">
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<tr>
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<th align="left"><a href="function_traits.html">Function object
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traits</a></th>
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<td valign="top"><tt>unary_traits<br>
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binary_traits</tt></td>
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<td valign="top">Used to determine the types of function objects' and
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functions' arguments. Eliminate the necessity for
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<tt>ptr_fun</tt>.</td>
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</tr>
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<tr>
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<th align="left"><a href="negators.html">Negators</a></th>
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<td valign="top"><tt>unary_negate<br>
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binary_negate<br>
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not1<br>
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not2</tt></td>
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<td valign="top">Based on section 20.3.5 of the standard.</td>
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</tr>
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<tr>
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<th align="left"><a href="binders.html">Binders</a></th>
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<td valign="top"><tt>binder1st<br>
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binder2nd<br>
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bind1st<br>
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bind2nd</tt></td>
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<td valign="top">Based on section 20.3.6 of the standard.</td>
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</tr>
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<tr>
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<th align="left"><a href="ptr_fun.html">Adapters for pointers to
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functions</a></th>
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<td valign="top"><tt>pointer_to_unary_function<br>
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pointer_to_binary_function<br>
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ptr_fun</tt></td>
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<td valign="top">Based on section 20.3.7 of the standard. Not required
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for use with this library since the binders and negators can adapt
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functions, but may be needed with third party adapters.</td>
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</tr>
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<tr>
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<th align="left"><a href="mem_fun.html">Adapters for pointers to member
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functions</a></th>
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<td valign="top"><tt>mem_fun_t<br>
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mem_fun1_t<br>
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const_mem_fun_t<br>
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const_mem_fun1_t<br>
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mem_fun_ref_t<br>
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mem_fun1_ref_t<br>
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const_mem_fun_ref_t<br>
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const_mem_fun1_ref_t<br>
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mem_fun<br>
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mem_fun_ref</tt></td>
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<td valign="top">Based on section 20.3.8 of the standard.</td>
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</tr>
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</table>
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<h3>Usage</h3>
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<p>Using these adapters should be pretty much the same as using the
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standard function object adapters; the only differences are that you need
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to write <tt>boost::</tt> instead of <tt>std::</tt>, and that you will get
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fewer headaches.</p>
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<p>For example, suppose you had a <tt>Person</tt> class that contained a
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<tt>set_name</tt> function:</p>
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<blockquote>
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<pre>
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class Person
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{
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public:
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void set_name(const std::string &name);
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// ...
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};
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</pre>
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</blockquote>
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<p>You could rename a bunch of people in a collection, <tt>c</tt>, by
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writing</p>
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<blockquote>
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<pre>
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std::for_each(c.begin(), c.end(),
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boost::bind2nd(boost::mem_fun_ref(&Person::set_name), "Fred"));
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</pre>
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</blockquote>
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<p>If the standard adapters had been used instead then this code would
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normally fail to compile, because <tt>set_name</tt> takes a reference
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argument. Refer to the comments in the <a href="binders.html#refref">binder
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documentation</a> to explain why this is so.</p>
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<h3>Compiler Compatibility</h3>
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<p>The header and <a href="test/function_test.cpp">test program</a> have been
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compiled with the following compilers:</p>
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<table border="1" cellpadding="5" summary="">
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<tr>
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<th>Compiler</th>
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<th>Comments</th>
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</tr>
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<tr>
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<td valign="top">Borland C++Builder 4 Update 2</td>
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<td valign="top">No known issues.</td>
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</tr>
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<tr>
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<td valign="top">Borland C++ 5.5</td>
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<td valign="top">No known issues.</td>
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</tr>
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<tr>
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<td valign="top">g++ 2.95.2</td>
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<td valign="top">No known issues.</td>
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</tr>
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<tr>
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<td valign="top">Microsoft Visual C++ Service Pack 3</td>
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<td valign="top">
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Compiler lacks partial specialisation, so this library offers little
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more than is provided by the standard adapters:
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<ul>
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<li>The <tt>call_traits</tt> mechanism is unable to prevent
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references to references, and so the adapters in this library will
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be usable in fewer situations.</li>
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<li>The <tt>function_traits</tt> mechanism is unable to determine
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the argument and result types of functions, therefore
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<tt>ptr_fun</tt> continues to be required to adapt functions.</li>
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</ul>
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</td>
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</tr>
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</table>
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<h3>Future Directions</h3>
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<p>This library's primary focus is to solve the problem of references to
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references while maintaining as much compatibility as possible with the
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standard library. This allows you to use the techniques you read about in
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books and magazines with many of today's compilers.</p>
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<p>In the longer term, even better solutions are likely:</p>
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<ol>
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<li>Several Boost members are working on expression template libraries.
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These will allow a more natural syntax for combining and adapting
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functions. As this is a new technology, it may be some time before it has
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matured and is widely supported by major compilers but shows great
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promise. In the meantime, the functional.hpp library fills the gap.</li>
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<li>The Standard Committee has recognised the problem of references to
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references occurring during template instantiation and has moved to fix
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the standard (see the <a href=
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"http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#106">C++
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standard core language active issues list</a>).</li>
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</ol>
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<h3>Author</h3>
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<p><a href="http://www.boost.org/people/mark_rodgers.htm">Mark Rodgers</a></p>
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<h3>Acknowledgements</h3>
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<p>Thanks to <a href="http://www.boost.org/people/john_maddock.htm">John Maddock</a> for
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suggesting the mechanism that allowed the function objects traits to work
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correctly. <a href="http://www.boost.org/people/jens_maurer.htm">Jens Maurer</a> provided
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invaluable feedback during the <a href=
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"http://www.boost.org/more/formal_review_process.htm">formal review process</a>.</p>
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<hr>
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<p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src=
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"../../doc/images/valid-html401.png" alt="Valid HTML 4.01 Transitional"
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height="31" width="88"></a></p>
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<p>Revised
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<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->02
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December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38510" --></p>
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<p><i>Copyright © 2000 Cadenza New Zealand Ltd.</i></p>
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<p><i>Distributed under the Boost Software License, Version 1.0. (See
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accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
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copy at <a href=
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"http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
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