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| 31 | </tr> |
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| 32 | </table> |
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| 33 | |
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| 34 | <h1>Function Pointer Adapters</h1> |
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| 35 | |
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| 36 | <p>The header <a href="../../boost/functional.hpp">functional.hpp</a> |
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| 37 | provides enhanced versions of both the function pointer adapters from the |
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| 38 | C++ Standard Library (§20.3.7):</p> |
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| 39 | |
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| 40 | <ul> |
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| 41 | <li><tt>pointer_to_unary_function</tt></li> |
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| 42 | |
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| 43 | <li><tt>pointer_to_binary_function</tt></li> |
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| 44 | </ul> |
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| 45 | |
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| 46 | <p>As well as the corresponding helper function template:</p> |
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| 47 | |
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| 48 | <ul> |
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| 49 | <li><tt>ptr_fun</tt></li> |
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| 50 | </ul> |
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| 51 | |
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| 52 | <p>However, you should not need to use the adapters in conjunction with the |
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| 53 | adapters in this library due to our use of <a href= |
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| 54 | "function_traits.html">function object traits</a>. You will however need to |
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| 55 | use them if your implementation fails to work properly with our traits |
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| 56 | classes (due to lack if partial specialisation), or if you wish to use a |
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| 57 | function object adapter from a third party.</p> |
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| 58 | |
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| 59 | <h3>Usage</h3> |
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| 60 | |
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| 61 | <p>If you need to use these adapters, usage is identical to the standard |
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| 62 | function pointer adapters. For example,</p> |
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| 63 | |
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| 64 | <blockquote> |
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| 65 | <pre> |
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| 66 | bool bad(std::string foo) { ... } |
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| 67 | ... |
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| 68 | std::vector<std::string> c; |
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| 69 | ... |
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| 70 | std::vector<std::string>::iterator it |
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| 71 | = std::find_if(c.begin(), c.end(), std::not1(boost::ptr_fun(bad))); |
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| 72 | </pre> |
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| 73 | </blockquote> |
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| 74 | |
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| 75 | <p>Note however that this library contains enhanced <a href= |
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| 76 | "negators.html">negators</a> that support function object traits, so the |
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| 77 | line above could equally be written</p> |
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| 78 | |
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| 79 | <blockquote> |
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| 80 | <pre> |
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| 81 | std::vector<std::string>::iterator it |
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| 82 | = std::find_if(c.begin(), c.end(), boost::not1(bad)); |
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| 83 | </pre> |
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| 84 | </blockquote> |
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| 85 | |
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| 86 | <h3>Argument Types</h3> |
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| 87 | |
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| 88 | <p>The standard defines <tt>pointer_to_unary_function</tt> like this |
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| 89 | (§20.3.8 ¶2):</p> |
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| 90 | |
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| 91 | <blockquote> |
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| 92 | <pre> |
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| 93 | template <class Arg, class Result> |
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| 94 | class pointer_to_unary_function : public unary_function<Arg, Result> { |
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| 95 | public: |
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| 96 | explicit pointer_to_unary_function(Result (* f)(<strong>Arg</strong>)); |
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| 97 | Result operator()(<strong>Arg</strong> x) const; |
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| 98 | }; |
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| 99 | </pre> |
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| 100 | </blockquote> |
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| 101 | |
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| 102 | <p>Note that the argument to <tt>operator()</tt> is exactly the same type |
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| 103 | as the argument to the wrapped function. If this is a value type, the |
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| 104 | argument will be passed by value and copied twice. |
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| 105 | <tt>pointer_to_binary_function</tt> has a similar problem.</p> |
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| 106 | |
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| 107 | <p>However, if we were to try and eliminate this inefficiency by instead |
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| 108 | declaring the argument as <tt>const Arg&</tt>, then if Arg were a |
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| 109 | reference type, we would have a reference to a reference, which is |
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| 110 | currently illegal (but see <a href= |
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| 111 | "http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/cwg_active.html#106">C++ core |
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| 112 | language issue number 106)</a></p> |
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| 113 | |
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| 114 | <p>So the way in which we want to declare the argument for |
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| 115 | <tt>operator()</tt> depends on whether or not the wrapped function's |
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| 116 | argument is a reference. If it is a reference, we want to declare it simply |
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| 117 | as <tt>Arg</tt>; if it is a value we want to declare it as |
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| 118 | <tt>const Arg&</tt>.</p> |
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| 119 | |
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| 120 | <p>The Boost <a href="../utility/call_traits.htm">call_traits</a> class |
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| 121 | template contains a <tt>param_type</tt> typedef, which uses partial |
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| 122 | specialisation to make precisely this decision. By declaring the |
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| 123 | <tt>operator()</tt> as</p> |
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| 124 | |
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| 125 | <blockquote> |
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| 126 | <pre> |
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| 127 | Result operator()(typename call_traits<Arg>::param_type x) const |
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| 128 | </pre> |
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| 129 | </blockquote> |
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| 130 | |
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| 131 | <p>we achieve the desired result - we improve efficiency without generating |
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| 132 | references to references.</p> |
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| 133 | |
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| 134 | <h3>Limitations</h3> |
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| 135 | |
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| 136 | <p>The call traits template used to realise this improvement relies on |
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| 137 | partial specialisation, so this improvement is only available on compilers |
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| 138 | that support that feature. With other compilers, the argument passed to the |
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| 139 | function will always be passed by reference, thus generating the |
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| 140 | possibility of references to references.</p> |
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| 141 | <hr> |
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| 142 | |
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| 143 | <p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src= |
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| 145 | height="31" width="88"></a></p> |
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| 146 | |
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| 147 | <p>Revised |
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| 148 | <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->02 |
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| 149 | December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38510" --></p> |
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| 150 | |
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| 151 | <p><i>Copyright © 2000 Cadenza New Zealand Ltd.</i></p> |
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| 152 | |
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| 153 | <p><i>Distributed under the Boost Software License, Version 1.0. (See |
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| 154 | accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or |
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| 155 | copy at <a href= |
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| 156 | "http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p> |
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| 157 | </body> |
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