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| 7 | <title>Boost Function Object Adapter Library</title> |
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| 14 | <td bgcolor="#FFFFFF"><img src="../../boost.png" alt="boost.png (6897 bytes)" width="277" height="86"></td> |
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| 15 | <td><a href="../../index.htm"><font face="Arial" color="#FFFFFF"><big>Home</big></font></a></td> |
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| 16 | <td><a href="../libraries.htm"><font face="Arial" color="#FFFFFF"><big>Libraries</big></font></a></td> |
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| 17 | <td><a href="../../people/people.htm"><font face="Arial" color="#FFFFFF"><big>People</big></font></a></td> |
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| 18 | <td><a href="../../more/faq.htm"><font face="Arial" color="#FFFFFF"><big>FAQ</big></font></a></td> |
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| 19 | <td><a href="../../more/index.htm"><font face="Arial" color="#FFFFFF"><big>More</big></font></a></td> |
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| 20 | </tr> |
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| 21 | </table> |
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| 22 | <h1>Improved Function Object Adapters</h1> |
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| 23 | <p>The header <nobr><a href="../../boost/functional.hpp">functional.hpp</a></nobr> |
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| 24 | provides enhancements to the function object adapters specified in the C++ |
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| 25 | Standard Library (sections 20.3.5, through to 20.3.8). The enhancements are |
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| 26 | principally possible due to two changes:</p> |
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| 27 | <ol> |
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| 28 | <li>We use the Boost <nobr><tt><a href="../utility/call_traits.htm">call_traits</a></tt></nobr> |
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| 29 | templates to avoid the problem of <a href="binders.html#refref">references |
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| 30 | to references</a>, and to improve the efficiency of <a href="mem_fun.html#args">parameter |
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| 31 | passing</a>.</li> |
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| 32 | <li>We use two <a href="function_traits.html">function object traits</a> class |
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| 33 | templates to avoid the need for <nobr><tt><a href="ptr_fun.html">ptr_fun</a></tt></nobr> |
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| 34 | with the adapters in this library.</li> |
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| 35 | </ol> |
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| 36 | <h3>Contents</h3> |
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| 37 | <p>The header contains the following function and class templates:</p> |
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| 38 | <table border="1" cellpadding="5"> |
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| 39 | <tr> |
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| 40 | <th align="left"><a href="function_traits.html">Function object traits</a> |
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| 41 | <td valign="top"><tt><nobr>unary_traits</nobr><br> |
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| 42 | <nobr>binary_traits</nobr></tt></td> |
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| 43 | <td valign="top">Used to determine the types of function objects' and |
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| 44 | functions' arguments. Eliminate the necessity for <nobr><tt>ptr_fun</tt></nobr>.</td> |
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| 45 | </tr> |
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| 46 | <tr> |
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| 47 | <th align="left"><a href="negators.html">Negators</a></th> |
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| 48 | <td valign="top"><tt><nobr>unary_negate</nobr><br> |
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| 49 | <nobr>binary_negate</nobr><br> |
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| 50 | <nobr>not1</nobr><br> |
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| 51 | <nobr>not2</nobr></tt></td> |
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| 52 | <td valign="top">Based on section 20.3.5 of the standard.</td> |
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| 53 | </tr> |
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| 54 | <tr> |
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| 55 | <th align="left"><a href="binders.html">Binders</a></th> |
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| 56 | <td valign="top"><tt><nobr>binder1st</nobr><br> |
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| 57 | <nobr>binder2nd</nobr><br> |
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| 58 | <nobr>bind1st</nobr><br> |
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| 59 | <nobr>bind2nd</nobr></tt></td> |
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| 60 | <td valign="top">Based on section 20.3.6 of the standard.</td> |
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| 61 | </tr> |
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| 62 | <tr> |
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| 63 | <th align="left"><a href="ptr_fun.html">Adapters for pointers to functions</a></th> |
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| 64 | <td valign="top"><tt><nobr>pointer_to_unary_function</nobr><br> |
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| 65 | <nobr>pointer_to_binary_function</nobr><br> |
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| 66 | <nobr>ptr_fun</nobr></tt></td> |
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| 67 | <td valign="top">Based on section 20.3.7 of the standard. Not required for |
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| 68 | use with this library since the binders and negators can adapt functions, |
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| 69 | but may be needed with third party adapters.</td> |
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| 70 | </tr> |
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| 71 | <tr> |
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| 72 | <th align="left"><a href="mem_fun.html">Adapters for pointers to member |
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| 73 | functions</a></th> |
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| 74 | <td valign="top"><tt><nobr>mem_fun_t</nobr><br> |
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| 75 | <nobr>mem_fun1_t</nobr><br> |
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| 76 | <nobr>const_mem_fun_t</nobr><br> |
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| 77 | <nobr>const_mem_fun1_t</nobr><br> |
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| 78 | <nobr>mem_fun_ref_t</nobr><br> |
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| 79 | <nobr>mem_fun1_ref_t</nobr><br> |
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| 80 | <nobr>const_mem_fun_ref_t</nobr><br> |
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| 81 | <nobr>const_mem_fun1_ref_t</nobr><br> |
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| 82 | <nobr>mem_fun</nobr><br> |
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| 83 | <nobr>mem_fun_ref</nobr></tt></td> |
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| 84 | <td valign="top">Based on section 20.3.8 of the standard.</td> |
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| 85 | </tr> |
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| 86 | </table> |
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| 87 | <h3>Usage</h3> |
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| 88 | <p>Using these adapters should be pretty much the same as using the standard |
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| 89 | function object adapters; the only differences are that you need to write <nobr><tt>boost::</tt></nobr> |
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| 90 | instead of <nobr><tt>std::</tt></nobr>, and that you will get fewer headaches.</p> |
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| 91 | <p>For example, suppose you had a <tt>Person</tt> class that contained a <nobr><tt>set_name</tt></nobr> |
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| 92 | function: |
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| 93 | <blockquote> |
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| 94 | <pre> |
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| 95 | class Person |
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| 96 | { |
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| 97 | public: |
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| 98 | void set_name(const std::string &name); |
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| 99 | // ... |
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| 100 | }; |
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| 101 | </pre> |
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| 102 | </blockquote> |
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| 103 | <p>You could rename a bunch of people in a collection, <tt>c</tt>, by writing</p> |
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| 104 | <blockquote> |
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| 105 | <pre> |
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| 106 | std::for_each(c.begin(), c.end(), |
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| 107 | boost::bind2nd(boost::mem_fun_ref(&Person::set_name), "Fred")); |
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| 108 | </pre> |
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| 109 | </blockquote> |
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| 110 | <p>If the standard adapters had been used instead then this code would normally |
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| 111 | fail to compile, because <tt><nobr>set_name</nobr></tt> takes a reference |
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| 112 | argument. Refer to the comments in the <a href="binders.html#refref">binder |
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| 113 | documentation</a> to explain why this is so.</p> |
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| 114 | <h3>Compiler Compatibility</h3> |
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| 115 | <p>The header and <a href="function_test.cpp">test program</a> have been |
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| 116 | compiled with the following compilers:</p> |
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| 117 | <table border="1" cellpadding="5"> |
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| 118 | <tr> |
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| 119 | <th>Compiler</th> |
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| 120 | <th>Comments</th> |
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| 121 | </tr> |
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| 122 | <tr> |
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| 123 | <td valign="top">Borland C++Builder 4 Update 2</td> |
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| 124 | <td valign="top">No known issues.</td> |
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| 125 | </tr> |
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| 126 | <tr> |
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| 127 | <td valign="top">Borland C++ 5.5</td> |
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| 128 | <td valign="top">No known issues.</td> |
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| 129 | </tr> |
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| 130 | <tr> |
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| 131 | <td valign="top">g++ 2.95.2</td> |
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| 132 | <td valign="top">No known issues.</td> |
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| 133 | </tr> |
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| 134 | <tr> |
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| 135 | <td valign="top">Microsoft Visual C++ Service Pack 3</td> |
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| 136 | <td valign="top">Compiler lacks partial specialisation, so this library |
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| 137 | offers little more than is provided by the standard adapters: |
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| 138 | <ul> |
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| 139 | <li>The <nobr><tt>call_traits</tt></nobr> mechanism is unable to prevent |
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| 140 | references to references, and so the adapters in this library will be |
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| 141 | usable in fewer situations.</li> |
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| 142 | <li>The <nobr><tt>function_traits</tt></nobr> mechanism is unable to |
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| 143 | determine the argument and result types of functions, therefore <nobr><tt>ptr_fun</tt></nobr> |
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| 144 | continues to be required to adapt functions. |
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| 145 | </ul> |
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| 146 | </td> |
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| 147 | </tr> |
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| 148 | </table> |
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| 149 | <h3>Future Directions</h3> |
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| 150 | <p>This library's primary focus is to solve the problem of references to |
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| 151 | references while maintaining as much compatibility as possible with the standard |
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| 152 | library. This allows you to use the techniques you read about in books and |
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| 153 | magazines with many of today's compilers.</p> |
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| 154 | <p>In the longer term, even better solutions are likely:</p> |
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| 155 | <ol> |
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| 156 | <li>Several Boost members are working on expression template libraries. These |
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| 157 | will allow a more natural syntax for combining and adapting functions. As |
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| 158 | this is a new technology, it may be some time before it has matured and is |
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| 159 | widely supported by major compilers but shows great promise. In the |
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| 160 | meantime, the functional.hpp library fills the gap.</li> |
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| 161 | <li>The Standard Committee has recognised the problem of references to |
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| 162 | references occurring during template instantiation and has moved to fix the |
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| 163 | standard (see the <a href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/cwg_active.html#106">C++ |
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| 164 | standard core language active issues list</a>).</li> |
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| 165 | </ol> |
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| 166 | <h3>Author</h3> |
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| 167 | <p><a href="../../people/mark_rodgers.htm">Mark Rodgers</a></p> |
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| 168 | <h3>Acknowledgements</h3> |
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| 169 | <p>Thanks to <a href="../../people/john_maddock.htm">John Maddock</a> for |
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| 170 | suggesting the mechanism that allowed the function objects traits to work |
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| 171 | correctly. <a href="../../people/jens_maurer.htm">Jens Maurer</a> provided |
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| 172 | invaluable feedback during the <a href="../../more/formal_review_process.htm">formal |
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| 173 | review process</a>. |
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| 174 | <hr> |
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| 175 | <p>Copyright © 2000 Cadenza New Zealand Ltd. Permission to copy, use, modify, |
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| 176 | sell and distribute this document is granted provided this copyright notice |
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| 177 | appears in all copies. This document is provided "as is" without |
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| 178 | express or implied warranty, and with no claim as to its suitability for any |
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| 179 | purpose.</p> |
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| 180 | <p>Revised 28 June 2000</p> |
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| 181 | |
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| 182 | </body> |
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| 183 | |
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| 184 | </html> |
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