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<meta name="GENERATOR" content="Microsoft FrontPage 4.0">
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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">
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<tr>
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<td bgcolor="#FFFFFF"><img src="../../c++boost.gif" alt="c++boost.gif (8819 bytes)" width="277" height="86"></td>
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<td><a href="../../index.htm"><font face="Arial" color="#FFFFFF"><big>Home</big></font></a></td>
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<td><a href="../../libraries.htm"><font face="Arial" color="#FFFFFF"><big>Libraries</big></font></a></td>
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<td><a href="../../people.htm"><font face="Arial" color="#FFFFFF"><big>People</big></font></a></td>
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<td><a href="../../more/faq.htm"><font face="Arial" color="#FFFFFF"><big>FAQ</big></font></a></td>
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<td><a href="../../more/index.htm"><font face="Arial" color="#FFFFFF"><big>More</big></font></a></td>
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</tr>
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</table>
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<h1>Improved Function Object Adapters</h1>
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<p>The header <nobr><a href="../../boost/functional.hpp">functional.hpp</a></nobr>
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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 <nobr><tt><a href="../utility/call_traits.htm">call_traits</a></tt></nobr>
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templates to avoid the problem of <a href="binders.html#refref">references
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to references</a>, 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> class
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templates to avoid the need for <nobr><tt><a href="ptr_fun.html">ptr_fun</a></tt></nobr>
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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">
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<tr>
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<th align="left"><a href="function_traits.html">Function object traits</a>
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<td valign="top"><tt><nobr>unary_traits</nobr><br>
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<nobr>binary_traits</nobr></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 <nobr><tt>ptr_fun</tt></nobr>.</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><nobr>unary_negate</nobr><br>
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<nobr>binary_negate</nobr><br>
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<nobr>not1</nobr><br>
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<nobr>not2</nobr></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><nobr>binder1st</nobr><br>
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<nobr>binder2nd</nobr><br>
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<nobr>bind1st</nobr><br>
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<nobr>bind2nd</nobr></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 functions</a></th>
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<td valign="top"><tt><nobr>pointer_to_unary_function</nobr><br>
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<nobr>pointer_to_binary_function</nobr><br>
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<nobr>ptr_fun</nobr></tt></td>
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<td valign="top">Based on section 20.3.7 of the standard. Not required for
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use with this library since the binders and negators can adapt functions,
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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><nobr>mem_fun_t</nobr><br>
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<nobr>mem_fun1_t</nobr><br>
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<nobr>const_mem_fun_t</nobr><br>
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<nobr>const_mem_fun1_t</nobr><br>
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<nobr>mem_fun_ref_t</nobr><br>
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<nobr>mem_fun1_ref_t</nobr><br>
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<nobr>const_mem_fun_ref_t</nobr><br>
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<nobr>const_mem_fun1_ref_t</nobr><br>
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<nobr>mem_fun</nobr><br>
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<nobr>mem_fun_ref</nobr></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 standard
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function object adapters; the only differences are that you need to write <nobr><tt>boost::</tt></nobr>
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instead of <nobr><tt>std::</tt></nobr>, and that you will get fewer headaches.</p>
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<p>For example, suppose you had a <tt>Person</tt> class that contained a <nobr><tt>set_name</tt></nobr>
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function:
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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 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 normally
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fail to compile, because <tt><nobr>set_name</nobr></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="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">
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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">Compiler lacks partial specialisation, so this library
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offers little more than is provided by the standard adapters:
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<ul>
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<li>The <nobr><tt>call_traits</tt></nobr> mechanism is unable to prevent
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references to references, and so the adapters in this library will be
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usable in fewer situations.</li>
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<li>The <nobr><tt>function_traits</tt></nobr> mechanism is unable to
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determine the argument and result types of functions, therefore <nobr><tt>ptr_fun</tt></nobr>
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continues to be required to adapt functions.
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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 standard
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library. This allows you to use the techniques you read about in books and
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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. These
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will allow a more natural syntax for combining and adapting functions. As
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this is a new technology, it may be some time before it has matured and is
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widely supported by major compilers but shows great promise. In the
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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 the
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standard (see the <a href="http://anubis.dkuug.dk/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="../../people/mark_rodgers.htm">Mark Rodgers</a></p>
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<h3>Acknowledgements</h3>
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<p>Thanks to <a href="../../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="../../people/jens_maurer.htm">Jens Maurer</a> provided
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invaluable feedback during the <a href="../../more/formal_review_process.htm">formal
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review process</a>.
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<hr>
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<p>Copyright © 2000 Cadenza New Zealand Ltd. Permission to copy, use, modify,
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sell and distribute this document is granted provided this copyright notice
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appears in all copies. This document is provided "as is" without
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express or implied warranty, and with no claim as to its suitability for any
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purpose.</p>
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<p>Revised 28 June 2000</p>
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</body>
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</html>
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